The Legio Mortis ("Death's Heads") are a Traitor Titan Legion of the Dark Mechanicum that repudiated their oaths to the Emperor of Mankind and followed the Warmaster Horus into the service of the Ruinous Powers during the Horus Heresy.
Even before the Horus Heresy, the princeps of the Legio Mortis were known for their cold and calculating demeanour, keeping meticulous kill tallies, and often filling their command decks with servitor-choirs that droned out an endless list of all of their victims.
This cacophony of the vanquished drove the Titan's crew to methodical acts of destruction, each fresh kill adding another name to the chant.
Originating in the pre-Imperial Age of Strife the Legio Mortis was, according to the legends of the Martian Lexmechanics, one of the Triad Ferrum Morgulus -- the three originating Titan Legions raised by the besieged Cult Mechanicum during the bloody wars to take back the Red Planet from the degenerate hordes of Cy-carnivora and mutant scavers that had overwhelmed Mars after the fall of the first Human interstellar civilisation at the end of the Age of Technology.
As was recorded in the iconography of their seal, the purpose of the Legio Mortis' creation was two-fold: firstly, to stand sentinel at the gates of the fabricator-general's domain as tireless guardians, and second, the annihilation of all life that would threaten the Machine Cult. As such, the Legio Mortis served as both sentry and exterminator.
Grim, dour and secretive, the Legio had gained a dark repute during the conquest of Mars as merciless and sinister that would only grow as the centuries passed, and it is now lost in time whether they gained their common name, "Death's Heads," from their iconography, the ritual onyx skull masks and sable robes their princeps wore, or their long practice of arraying the skeletal remains of those they had slain across the armoured pediments of their war machines as trophies.
Bound they were by rite and tradition in personal loyalty to the office of the fabricator-general of Mars, and many times were they used both against their enemies in war (for Mars was not truly pacified until the coming of the Emperor), and as a weapon to maintain the fabricator-general's authority among the tech-priests and punish recalcitrant magi and even other Titan Legions when needed.
This history garnered them few friends or allies and their long-standing rivalry with their fellow members of the Triad Ferrum Morgulus, most notably the Legio Ignatum, and their contempt for the "upstart" Titan Legions of other Forge Worlds than Mars was well-known.
Their reputation as an infamously fractious and insular force -- whose allegiance many within the Imperium's military staff believed remained solely to the fabricator-general and to the Emperor not at all -- resulted in the tragic irony of the Legio Mortis ' assignment to the personal flag of the Primarch Horus Lupercal of the Luna Wolves Legion for the purpose of guaranteeing and deepening the Titan Legion's loyalty to the Imperium.
Notwithstanding smaller battlegroups assigned to the Legio Mortis forge-fortress on Mars and to the blockade of the Uhn Holdfast, the bulk of the Titan Legion's active strength was assigned as the Titan backbone of the 63rd Expeditionary Fleet under Horus.
With that charismatic and deadly primarch they fought in over one hundred campaigns in the later decades of the Great Crusade, and the ashes of countless worlds and the bones of scores of different alien races adorned their god-engines.
During these years of blood and battle a new loyalty was indeed forged for the Legio Mortis, but it was not to the distant Emperor of Mankind but rather to Horus, and they became pledged to his standard above all, and followed him willingly into treachery.
Infrequently, those forces of the Legio Mortis that remained upon the Red Planet rather than accompanying Horus were known to depart at the behest of the fabricator-general, unleashed upon rebellious elements of the expanding Mechanicum or technological bastions that resisted the transfer of control to Mars. In this role, supported by the Pact of Morbidia, the Legio Mortis earned its reputation as callous and grim, the heavy tread of its god-engines crushing civilisations beneath them for the temerity of questioning a path not dictated by the fabricator-general.
Of all the Titan Legions that served within the Imperium, few save the Legio Mortis claimed the dubious honour of eradicating a Titan Legion in its entirety, at least until the bitter days of the Horus Heresy. In this did the Death's Heads revel, for it was upon the field of battle, as god-engine duelled god-engine, that the Titan Legion proved its self-assigned role as greatest amongst the Collegia Titanica.
Ironically, the Death's Heads would become corrupted by the Chaos God of decay, Nurgle -- a deity whose undead servants they had once fought against on Davin's moon, where Horus had been mortally wounded.
After the Warmaster's "miraculous" recovery from his wounds following his corruption by the Chaos Gods in the Temple of the Serpent Lodge, he began his rebellion against the Imperium by unleashing Life-eater Virus Bombs upon the world of Isstvan III, seeking to destroy elements of the Traitor Legions that were still steadfastly devoted to the Emperor of Mankind.
Elements of the Titan Legion had already descended to the surface where what few elements of the Legio that were not trusted were quickly purged.
It was to the Legio Mortis in their sealed war engines that Horus' plan had given the coup-de-grâce to the few Loyalist Astartes expected to survive the initial virus bombardment, but as matters transpired, what was to be an extermination to scour the survivors evolved into a wholesale battle.
Horus ordered the full Titan Legion deployed to smash the husks of the Isstvanian cities and break the Loyalists' resistance, and for the first time the Legiones Astartes met the Adeptus Titanicus in open conflict.
Some claim that mutated strains of the Life-Eater virus brought about their final corruption, while others say that the madness wrought by the sight of billions of putrefying corpses turned the Legio's faces to Chaos.
When the Horus Heresy became a full-blown insurrection, on Mars the traitorous Fabricator-General Kelbor-Hal of the Mechanicum decided to take the Warmaster's side in the conflict. The Legio Mortis supported the fabricator-general, being one of the first Titan Legions to fight in the service of the Ruinous Powers.
Open hostilities erupted all over the Red Planet as the civil war within the Mechanicum that became known as the Schism of Mars began. Loyalist and Traitor forces alike brought forth forbidden technologies to use against one another.
The Dark Mechanicum-aligned Legio Mortis eventually fought the Legio Tempestus in open warfare near Mars' Magma City, but these contingents of Death's Heads Titans were wiped out by the Loyalist Titan Legion that defended the forge city.
For the remainder of the Horus Heresy, the Legio Mortis proved a constant companion to the Warmaster and his forces. From Isstvan V and the world of Molech to the Beta-Garmon System and the walls of the Emperor's palace on Terra itself, the god-engines of the Legio Mortis brought ruin and death to all assembled beneath the Imperium's banner.
Records from innumerable battles during the Horus Heresy report the presence of Death's Heads Titans, yet their prolific acts of treachery did little to diminish the strength of the Legio Mortis, for the Warmaster ensured they remained well-supplied.
By the time the Legio Mortis landed on Terra to take part in the siege of the Imperial Palace during the Siege of Terra, its Titans had been warped and mutated beyond all recognition -- their adamantium hulls pockmarked and bubbling with fetid vapours.
The twisted flesh and metal of great tentacles lashed to and fro, spiked tails whipping about. The once-noble skull-faced heads adorning the Legio's Titans had now been transformed by the eldritch powers of Chaos into rancorous visages of drooling Daemons filled with malice, their engines roaring like angry beasts.
The Warmaster granted the Legio Mortis the singular honour of breaching the outer walls of the Imperial Palace. Using Wreckers, Power Rams, Warp Missiles and their own fiendish bravery they managed to accomplish this near-suicidal task despite suffering the loss of over thirty Titans in one evening of intense fighting.
The walls of the Imperial Palace finally broke under the onslaught by the Death's Heads Titan Legion -- the huge Warlord-class Titans of the Legion smashed their way through the last few metres of the bastion and unleashed a flood of Traitors into the inner courtyard of the Palace.
But despite their efforts, ultimately the Siege of Terra failed and Horus was laid low by the Emperor Himself. The remnants of the Legio Mortis fled from Terra hunted and pursued, system by system, to the Eye of Terror.
There, where the Warp and realspace overlap, time has flowed differently for the Death's Heads. They remain trapped in servitude to the Ruinous Powers, fighting the Long War against the forces of the Corpse Emperor that began ten millennia before, building their strength and testing the Imperium's defences for the time when they shall return and wreak terrible vengeance on all living things for the defeat of their beloved Warmaster.
- Accatran Compliance (Unknown Date.M30) - After the events of the campaign to bring Accatran into Imperial Compliance, the Legio Venator could no longer be trusted to fight alongside the Legio Mortis without the threat of internecine conflict between the two rival Titan Legions.
- Ullanor Crusade (999.M30-000.M31) - The Ullanor Crusade was a vast Imperial assault on the Ork empire of the Overlord Urrlak Urruk during the Great Crusade of the 31st Millennium. The capital world of this Greenskin empire, and the site of the final assault, lay in the Ullanor System. The crusade included the deployment of 100,000 Space Marines, 8,000,000 Imperial Army troops, and thousands of Imperial starships and their support personnel. The Luna Wolves Legion spearheaded the assault with Titans from the Legio Mortis providing heavy support. Striking directly at the heart of the Ork empire, this force succeeded in killing the Overlord, thus casting the Greenskin horde into disarray and unleashing infighting within its ranks. In recognition of this great victory, Horus was elevated to the esteemed rank of supreme commander of the Imperial armed forces and granted the title of Imperial Warmaster.
- Lament of Shedim (Unknown Date.M31) - This was an Imperial Compliance action carried out against the Craftworld Aeldari upon the arid world of Anark Zeta between the forces of Craftworld Mór-ríoh'i and the forces of the Great Crusade, which included detachments from no fewer than four Titan Legions: the Legio Mortis, Legio Fureans, the Legio Atarus and the Legio Osedax. Overall command fell to Horus Lupercal, most-favoured son of the Emperor and Primarch of the Luna Wolves Legion. During the apocalyptic battle, the Legio Atarus was cruelly used, and without their foreknowledge, they were sent to probe the enemy lines of the Aeldari, and were subsequently destroyed. The grievous losses suffered by the Firebrands due to the Legio Mortis and the Legio Fureans' refusal to fight alongside them due to internal Mechanicum politics, was a grievous injury not soon forgotten.
- Cleansing of Murder (One-Forty-Twenty) (ca. 001.M31) - The world of Urisarach (nicknamed by the Space Marines who fought on it as Murder) was the twentieth world that was first encountered by the 140th Expeditionary Fleet, which was composed of three companies from the IX Space Marine Legion, the Blood Angels, commanded by Captain Khitas Frome. This deadly world acquired its nickname of "Murder" from the last transmission received from Captain Frome before the failed Imperial expedition was massacred by the local alien population -- a deadly arachnoid race dubbed the Megarachnids. Eventually it took a combined strike-force composed of elements from the Blood Angels, Emperor's Children, and Luna Wolves Space Marines as well as a Legio Mortis Titan battlegroup to initiate a full-scale assault that drove back the ultra-violent xenos.
- Assault on Davin's Moon (004.M31) - At the behest of Chaplain Erebus of the Word Bearers Legion, Warmaster Horus and his 63rd Expeditionary Fleet were requested to return to the Feral World of Davin. Erebus reported that the Imperial Governor of Davin -- Commander Eugen Temba -- had gone Renegade after he and his forces had gone to the moon in an attempt to bring the Davinite tribes there into compliance. Unfortunately for Commander Temba, the parley with the tribespeople turned violent, and he and his men were struck down by a sorcerous attack that had twisted their loyalty to the Imperium. The Renegade Commander and his forces took refuge on Davin's moon which they soon transformed into a Nurgle-corrupted swamp land. Supported by Titans from the Legio Mortis, the Warmaster led the assault force personally. It was here that the Imperial forces were first confronted by a horde of reanimated corpses of Plague Zombies which was all that remained of Temba's Imperial Army garrison. Ironically, the Death's Heads would later become corrupted themselves by the Chaos Lord of Decay, Nurgle. They did not know that they were fighting against the deity in the form of hundreds of disease-ridden Plague Bearers. It was during the Warmaster's confrontation with the daemon-possessed Temba that he would fall, gravely wounded by the mysterious Chaotic sword known as the Kinebrach Anathame.
- Razing of Prospero (004.M31) - Though their involvement now largely fails to figure in most official records pertaining to the destruction of the Thousand Sons Legion homeworld of Prospero, the Imperial Censure Host included in its ranks a discreet body of forces which can be described accurately as the Warmaster Horus' servants, which included two full maniples and one demi-maniple of the Legio Mortis. Commanded by the highly decorated officer Princeps Senioris Maldis Drane, the Legio Mortis was ordered to stand down by the order of Leman Russ, Primarch of the Space Wolves Legion, and supreme commander of the Censure Host. He believed that the Emperor's orders were for the Legiones Astartes to carry out the vengeance of the Emperor upon their brothers, and the impersonal goliaths of the Legio Titanicus had no right of involvement in that judgement. As the fates would have it, the Legio Mortis contingent would be almost wiped out against a foe they may have been there to confront all along in the shadow of the Censure: the forces of the Zhao-Arkhad Mechanicum sub-fane, the Titans of Legio Xestobiax, far beyond the precincts of the city of Tizca. The evidence of the Legio Mortis being present as part of the Censure Host sent to Prospero has largely been lost to history. This is not only because their involvement was tangential to the Battle of Tizca, but also because the vaunted Legio Mortis suffered a bitter defeat at the hands of a brother Titan Legion considered by Mars to be a degraded imitation of the Omnissiah's ideal. That the defeat of the Legio Mortis force was only prevented from becoming out-right obliteration by the intervention of the shadowed Ordo Sinister was enough to pull a final shroud of secrecy over the debacle.
- Istvaan III Atrocity (ca. 007.M31) - The campaign known in later times as the Istvaan III Atrocity began when the Imperial Planetary Governor of Istvaan III, Vardus Praal, had been corrupted by the Chaos God Slaanesh whose cultists had long been active on the world. In 205.M31, Praal declared his world to be independent from the Imperium. Unaware of the change in the Warmaster's loyalties, the Council of Terra charged Horus with the retaking of that world, primarily its capital, the Choral City. When the Imperium finally gathered their forces, the Sons of Horus, Death Guard, Emperor's Children and the World Eaters Legions arrived in the Istvaan System, mustering in orbit above the doomed planet. Under the guise of putting down the rebellion against Imperial Compliance, Horus hatched the plan by which he would destroy all the remaining Loyalist elements of the Legions under his command. He purposely gathered those Astartes who were not loyal to him and used them in the assault on the non-compliant world. With elements of the Legio Mortis Titan Legion in support, the invasion of Istvaan III then began in earnest. One of these terrible war machines was the infamous Imperator-class Titan, the Dies Irae (Day of Wrath), which had already gained a fearsome reputation amongst the Astartes of the 63rd Expeditionary Fleet. At the moment of victory and the capture of the Choral City the Loyalist Astartes involved in the assault were betrayed when a cascade of terrible virus bombs fell onto the world, launched by the Warmaster's orbiting fleet. Caught in the deadly bombardment, the Princeps of the Dies Irae had the behemoth sealed against the deadly viral vapours, as his crew watched in horror as Istvaan III died around them, including many of the Loyalist Death Guard Astartes they had been supporting only moments before. Without the benefit of cooling systems and sufficient ventilation they were very nearly overcome by the intense heat when the secondary firestorm erupted. When the flames finally died down the Dies Irae went on to attack and help exterminate the remaining Loyalist forces within the ruins of Choral City.
- Schism of Mars (007.M31) - When open civil war erupted all over the Red Planet as the civil war within the Adeptus Mechanicus known as the Schism of Mars began, the Legio Mortis had already pledged their support to the Fabricator General and his co-conspirators of the Dark Mechanicum. Sallying forth from their fortress at Pavonis Mons, the Legio Mortis clashed with the Loyalist Titan Legion called the Legio Tempestus in open warfare near Magma City, but ultimately, these contingents of Death's Heads Titans were wiped out by the Loyalist Titan Legion.
- Ruin of Maedran (008.M31) - This battle of the Horus Heresy saw the Imperial colony world of Maedran on the edge of the Segmentum Solar become savagely contested between Loyalist and Traitor forces. Loyalist Titans of the Legio Gryphonicus, Legio Metalica, and Legio Destructor clashed with the Traitors of the Legio Mortis, Legio Argentum, and Legio Vulturum. One of the largest Titan battles of the early Heresy era, more than 200 god-engines clashed as Maedran was ultimately reduced to ruin. Ultimately heavy losses were incurred by both sides and neither combatant was able to claim true victory. In the aftermath, hatred between the Legio Gryphonicus and the Legio Mortis only intensified.
- Battle of Molech (008.M31) - The Legio Mortis was one of four Traitor Titan Legions that served at the Battle of Molech, providing fire support to the forces of the Warmaster Horus in his attempt to claim the planet and its secret Warp Gate for his own.
- Battle of Tallarn (010-011.M31) - The Legio Mortis is known to have fought in various engagements during the Horus Heresy, including the infamous Battle of Tallarn. During the Heresy in the early 31st Millennium, the Iron Warriors Traitor Legion, intending to destroy all potential resistance before landing, virus-bombed the world of Tallarn, rendering it an inhospitable desert. Only then did the Chaos Space Marines land. Tallarn's few survivors emerged from their underground bunkers to stop the Iron Warriors' invasion. Soon, reinforcements from both sides arrived, including the Legio Astorum Titan Legion. It soon became painfully clear to the Imperial forces the futility of fighting over a destroyed planet, but by then there was no turning back. Though it was not known at the time, the Iron Warriors were motivated to attack the world beyond their mere appetite for destruction because Tallarn was the site of a hidden and potentially powerful Chaos artefact. The poisoned environment of the world made it impossible for infantry troops to operate outside of their protective shelters, and so the only available option was to make use of armoured vehicles and the support of the Titan war machines of the Legio Astorum. The Legio Mortis fought their hated Loyalist kin from the Warp Runners, while the largest tank battle in human history erupted on the surface of Tallarn, involving some 10 million armoured vehicles. By the end of this brutal conflict, the Iron Warriors were defeated, and the wreckage of over a million tanks littered the sands.
- Siege of Terra (014.M31) - The Legio Mortis took part in numerous engagements during the dark days of the Horus Heresy, but their most notable action occurred when they took part in the Siege of Terra and the assault on the Imperial Palace. Horus commanded the Death's Head Titan Legion to break the walls. The fell Titans began to demolish entire sections of the bastion. The defenders inflicted dreadful carnage upon them - three, four then five massive Titans were destroyed in as many minutes. The Chaos Titans would not relent and they kept hammering away at the walls. The walls of the Emperor's Palace finally broke under the onslaught of the Death's Head Titan Legion -- its huge Warlord Titans smashed their way through the last few metres of the bastion and unleashed a flood of Traitors into the inner courtyard of the Palace. But despite their efforts, ultimately Horus was laid low by the Emperor himself and the Forces of Chaos were forced to flee into the Eye of Terror.
- Siege of Hydra Cordatus (999.M41) - During the 13th Black Crusade, an Iron Warriors Warsmith, known simply as "The Warsmith" (Barban Falk), laid siege to the world of Hydra Cordatus in an attempt to steal samples of Imperial Fists' gene-seed from the Adeptus Mechanicus fortress located on the planet. This Warsmith's warband of Iron Warriors was supported by a battlegroup of Chaos Titans from the Legio Mortis, including the infamous Imperator-class Titan Dies Irae. Three Loyalist Warlords, two Reavers and two Warhound-class Titans drawn from the Legio Ignatum were stationed on the planet to provide support for the Mechanicus garrison forces. The Imperial defenders fought back bravely and many of the Traitor Titans, including the Dies Irae, were destroyed with the support of the Legio Ignatum 's God-Machines. Unfortunately, the Warsmith eventually conquered the world and captured the Imperial Fists gene-seed, which would be used to create new generations of Iron Warriors Chaos Space Marines, a victory for which this Warsmith was raised by the Ruinous Powers to become a Daemon Prince.
- Aquila Ignis (Imperator-class Titan) - The Aquila Ignis was an Imperator-class Emperor Titan constructed in the Daedalia Forge Yards far to the south of Tharsis on Mars. The Aquila Ignis fought during the Schism of Mars against Loyalist elements from the Legio Tempestus in the battle for Magma City, where it was destroyed in the heavy combat.
- Dies Irae (Imperator-class Titan) - The Dies Irae was an Imperator-class Titan which was attached to the 63rd Expeditionary Fleet under the command of the Warmaster Horus. This infamous Titan was present during the opening days of the Horus Heresy at Istvaan III, when it followed Horus' orders and helped exterminate the remaining Loyalist Astartes of the Traitor Legions present on that cursed world after they were virus-bombed by their comrades from orbit. The Dies Irae was also present during all of the major ground assaults towards the end the rebellion. The Dies Irae finally met its fate ten millennia later during the siege of Hydra Cordatus in 999.M41 during the 13th Black Crusade when it served as part of an Iron Warriors attack force. When it was confronted by Imperial Titans from the Collegia Titanica, the Dies Irae was critically damaged when a Loyalist Warlord-class Titan detonated its plasma reactor when it was in close proximity to the Traitor Titan, which damaged the ancient Imperator-class Titan's armour. Another Warlord Titan then penetrated the Dies Irae's Void Shields and compromised external carapace with its Battle-Claw and crushed the Chaos-tainted Titan's plasma reactor. The resulting explosion wiped the Dies Irae's ancient taint from the face of the galaxy forever.
- Excoriator Vita (Warmaster Iconoclast-class Titan) - After its founding, the Legio Mortis was often used as the instrument of the fabricator-general's will. Thus, it is unsurprising that the Titan Legion, long-favoured by the ruling hierarchy of Mars, seemingly possessed the largest stock of Warmaster and Warmaster Iconclast pattern Titans within its vaults; collated estimates point to the Legio Mortis fielding between three and eight unique Warmaster and Warmaster Iconoclast Titans during the Horus Heresy. Of those, Excoriator Vita has the most surviving recorded data within numerous pict-captures recovered from the ruins of the Fortress World of Cerebiaus III. The partially corrupted footage recovered from the planet depicts the Iconoclast's efficient dismantling of the planetary capital's defensive lines, communication network and a maniple of Legio Ignatum Warhounds garrisoned within it. Cerebiaus III was not the only Loyalist stronghold to feel the wrath of Excoriator Vita, with the surrounding star systems bearing many scars marking the march of this god-engine.
- Breath of Thunder (Warlord-class Titan) - Fought with the Traitor Legions during the Horus Heresy at the Drop Site Massacre on Isstvan V; destroyed by the Legio Atarus Reaver-class Titan Red Naga.
- Cadavaris Perdita (Warlord-class Titan) - Command Titan of the Legio Mortis detachment sent by the Warmaster Horus alongside the Imperial Censure Host to Prospero and god-machine of Princeps Senioris Maldis Drane. The Warlord-class Titan Cadavaris Perdita was a storied veteran of the Great Crusade, having fought for many standard years alongside the Sons of Horus Legion and the 63rd Expeditionary Fleet. The attack on the Zhao-Arkhad sub-forge was to be Maldis Drane's last battle, however, as he was slain when his Titan was decapitated by the Legio Xestobiax Reaver-class Titan Sleepless Watcher. Yet the carcass of the venerable Warlord was recovered and re-commissioned by the Legio Mortis, but ever afterwards retained the shame of its defeat and was considered something of a pariah among the Death's Heads. Mentioned as Aeterna Virtus in the Prospero Censure Host list.
- Death's Head (Warlord-class Titan) - Participated in the assault on Davin's moon during the Great Crusade.
- Harbinger of Woes (Warlord-class Titan) - Legio Mortis Warlord Titans such as Harbinger of Woes were a common sight on the battlefields of the Beta-Garmon cluster during the Battle of Beta-Garmon. The close association the Legio Mortis had with Horus Lupercal during the Great Crusade continued during the Horus Heresy, with the Death's Heads participating in some of the most influential conflicts of the civil war. At Beta-Garmon, many Legio Mortis Titans added significant numbers of god-engine kills to their honour banners, with Harbinger of Woes claiming at least nine confirmed kills in the two standard years prior to Beta-Garmon's fall to the Traitors. Later images depict this Warlord Titan bearing numerous battle trophies from the conflict, including armour ripped from the wreckage of Legio Solaria and Legio Ignis Titans it had destroyed.
- Ire Incarnatus (Warlord-class Titan)- Fought with the Emperor's Children Traitor Legion during the Horus Heresy in the Tarren Suppression.
- Molossus (Warlord-class Titan)
- Penumbral Reaper (Warlord-class Titan) - Fought with the Sons of Horus Traitor Legion during the Horus Heresy during the Battle of Molech.
- Phantom Lord (Warlord-class Titan) - Fought with the Sons of Horus Traitor Legion during the Horus Heresy at the Battle of Molech.
- Manus Mortis (Warlord-class Titan) - A siege Warlord-class Titan that fought in the Great Crusade and the Battle of Terra. Present during the siege of the Imperial Palace.
- Tantus Abolitorus (Warlord-class Titan) - A Warlord of sinister reputation, Tantus Abolitorus was one of the eldest and most honoured Titans of the Legio Mortis, its battle-honours and kill-marks reputedly dating back to eternity. Located on Mars during the dire events of the Horus Heresy, Tantus Abolitorus fought in the early days of the Schism of Mars levelling the tower-perceptory of the Adeptus Astartes in the early hours of the civil war to ensure that no Space Marine would fight on the Loyalist side.
- Tellum Ire (Warlord-class Titan)
- Xestor's Sword (Warlord-class Titan) -Xestor's Sword fought with the Sons of Horus Traitor Legion during the Horus Heresy at the Battle of Molech.
- Black Consul (Reaver-class Titan)
- Dread Wake (Reaver-class Titan) - Fought with the Sons of Horus Traitor Legion during the Horus Heresy at the Battle of Molech. Participated in the Istvaan III Atrocity. It was part of War Maniple Ferrum Mori.
- Ferrum Mori (Reaver-class Titan) - Ferrum Mori participated in the Istvaan III Atrocity. It was Lead Battle-Titan of War Maniple Ferrum Mori.
- Hand of Ruin (Reaver-class Titan) - Hand of Ruin fought with the Sons of Horus Traitor Legion during the Horus Heresy at the the Battle of Molech. It also participated in the Istvaan III Atrocity. It was a member of War Maniple Ferrum Mori.
- Iron Baron (Reaver-class Titan) - Iron Baron participated in the First Battle of Rhetori III during the Horus Heresy.
- Myrmidon Rex (Reaver-class Titan) - Myrmidon Rex participated in the Istvaan III Atrocity. It also fought with the Sons of Horus Traitor Legion during the Horus Heresy at the the Battle of Molech. It was part of War Maniple Ferrum Mori.
- Mortmain (Reaver-class Titan) - Mortmain participated in the Istvaan III Atrocity. It was part of War Maniple Ferrum Mori.
- Varcarnerix (Reaver-class Titan) - The Varcarnerix was a truly ancient war engine, whose armoured core, although rebuilt and repaired many times, dated back to the ancient wars of Mars during the Age of Strife. Taking part in the Razing of Prospero as a part of the Legio Mortis contingent assigned to the Imperial Censure Host, it was badly damaged and forced into retreat early in the battle against the unleashed Taghmata of the Arkhadine sub-forge, having borne the brunt of the enemy battle-automatas' and Krios tanks' barrage. It was this withdrawal that enabled its survival. As a result, it did not face the fury of the Legio Xestobiax, nor suffer from the arrogance of the Legio Mortis' attack pattern against them.
- Vulkus Tor (Reaver-class Titan) - Destroyed by Orks on Ullanor during the Great Crusade.
- Blood Wolf (Warhound-class Titan) - Blood Wolf participated in the Istvaan III Atrocity. It was a member of War Maniple Ferrum Mori.
- Carrion Crow (Warhound-class Titan) - Carrion Crow participated in the Istvaan III Atrocity. It was a member of War Maniple Ferrum Mori.
- Scar of Honour (Warhound-class Titan) - One of the few Scout Titans attached to the Legio Mortis detachment during the Fall of Prospero, the Scar of Honour was a relatively recent creation of the Martian forges, its livery as yet unadorned by heraldic victory markings. Rather than deploy his Warhounds as a skirmish line to probe the defences of his foe when approaching the surface ruins of the Zhao-Arkhad sub-fane, Princeps Senioris Drane, in command of the Legio Mortis during planetfall, instead deployed them as outriders out on the extreme flanks of their approach, expecting them to hunt down the fleeing remnants of a broken foe. As a result, the Warhounds were the last to engage the Xestobiax and the last to be destroyed.
- Sothis (Warhound-class Titan) - Sothis participated in the Istvaan III Atrocity. It was a member of War Maniple Ferrum Mori.
- Tenebris Fedelis (Warhound-class Titan) - Tenebris Fidelis fought with the Traitor Legions during the Horus Heresy at the Drop Site Massacre on Istvaan V.
- Tellum Ire (Warhound-class Titan) - Tellum Ire participated in the Battle of Beta-Garmon during the Horus Heresy.
- Venator (Warhound-class Titan) - The Venator was a member of the Death's Head Scouts, a Warhound Titan battlegroup that fought during the Horus Heresy. It was the excesses committed by Senior Princeps Esau Turnet's Death's Head Scouts during the course of the Horus Heresy which were directly responsible for the Imperium later terminating the practice of organising Scout Titans (such as the Warhound) into battlegroups. By the order of the High Lords of Terra they were limited to operating singly or in pairs, so that the horror of civilian massacres, perpetrated by packs of Warhound-class Titans during the Battle of Terra, might never again be visited upon Mankind.
- Camulos - Senior princeps, commander of the Legio Mortis' forces on Mars at the Legion's fortress located at Pavonis Mons. Camulos led the Titans of his Legio against the Loyalist Titans of the Legio Tempestus in open warfare at the Battle of Magma City during the Schism of Mars.
- Maldis Drane - Madis Drane was a Princeps Senioris and the commander of the Warhound-class Titan Cadavaris Perdita. He was the commander of the Titan contingent of the Legio Mortis sent by the Warmaster Horus to fight alongside the Imperial Censure Host during the Razing of Prospero. A storied and highly decorated veteran, Drane was one of Horus' favoured, quite beyond his loyalty to Mars, and this no doubt was one of the reasons the Warmaster had contrived to have the forces at the Princeps' command attached to the Censure Host under the Warmaster's own authority. The attack on the Zhao-Arkhad sub-forge was to be Maldis Drane's last battle, however, as he was slain when his Titan was decapitated by the Legio Xestobiax Reaver-class Titan Sleepless Watcher.
- Esau Turnet - Esau Turnet was a Princeps Senioris and the commander of the Imperator-class Emperor Titan Dies Irae during the Horus Heresy. The Senior Princeps had already repudiated his oaths to the Emperor and sworn allegiance to the Warmaster by the time the events of Istvaan III occurred. Turnet was privy to what Horus had planned for the Loyalist Astartes on the surface of the doomed planet.
- Jonah Aruken - Aruken was the Moderati Primus of the Imperator-class Titan Dies Irae during the Horus Heresy. When the Warmaster enacted the opening phase of his rebellion on Istvaan III, he was present on the Dies Irae. After the virus-bombing and secondary firestorm, Senior Princeps ordered the Titan's weapons to be reactivated and to fire upon the Loyalist Death Guard Astartes. Horrified, his friend and fellow Moderati Titus Cassar attempted to stop the Senior Princeps from carrying out his unlawful orders. Seeing his future ambitions to command his own Titan being sabotaged by his friend Titus, Jonah shot and killed his fellow Moderati. He then carried out Senior Princeps Turnet's orders and proceeded to fire upon the loyalists.
- Titus Cassar - Moderati Primus of the Imperator-class Titan Dies Irae during the Horus Heresy. He was secretly a member of the Emperor-worshiping cult known as the Lectitio Divinitatus, which believed that the Emperor was a divine being and the God of Mankind. He was often ridiculed by his friend and fellow Moderati Jonah Aruken for his beliefs. Titus was present during the opening days of the Warmaster's rebellion at Istvaan III. He watched in horror as the planet was virus-bombed and died and the subsequent firestorm that followed. When ordered by Senior Princeps Turnet to reactivate their weapons and open fire on the Death Guard Astartes that they had supported only moments before, Titus attempted to stop him with a pistol, but was killed by his friend Jonah Aruken.
Thanks to its ancient history and its close ties to the fabricator-general of Mars and his coterie, the Legio Mortis was one of the numerically largest and best-equipped of all of the Titan Legions at the outbreak of the Horus Heresy, recorded as a Primus (Maxis)-grade Titan Legion.
Conservative estimates place their disposition at or around 200 Titans in active service, as well as having 2 battalions of specialised support armour, 5 Skitarii Auxilia Legions and a close air-support phalanx of 9 squadrons bound to their service.
Only the Legio Destructor is believed to have been able to put more Battle Titans into the field at the civil war's outbreak. In particular, the Legio Mortis was known to possess a strong contingent of the most powerful classes of Titan: the Imperator, Nemesis and at least one Apocalypse Pattern war engine.
The allied Knight houses whose Knights often served alongside the Legio Mortis include House Morbidia, House Ju'll and House Senica. During the Battle of Beta-Garmon the Legio Mortis was supported by House Devine, House Gotrith, House Oroborn, House Vextrix and House Xerathon.
The Pre-Heresy colours of the Legio Mortis were black, red, silver and gold. Now that they are the corrupted followers of Nurgle, their Titans no longer resemble the once-proud war machines of the past.
They display the outwards signs of their allegiance to the Plague God, and are covered in all manner of putrescence, decay and filth.
The Legio Mortis badge is a skull centred in front of a gate with chains dangling from it. Half of the Cult Mechanicus cog symbol sits atop the gate. This symbolises the Legio's ancient mission of both standing sentinel at the fabricator-general's gates as the personal protectors of their domain, and annihilating all life that would threaten the Machine Cult, for the Legio Mortis served as both sentry and exterminator. Before the Horus Heresy, the symbol of the Collegia Titanica was centred on the forehead of the skull, an emblem that was removed following the Legio's fall to Chaos.
A variant of the Legion's badge is a horned death's head in front of a red-outlined blue delta (triangle) on a field of black. Below the death's head are rows of skulls, symbolising the Legio's skill at bringing death to its enemies.
- Adeptus Titanicus (1st Edition), pg. 59
- The Horus Heresy - Book One: Betrayal by Alan Bligh (Forge World Series), pp. 138-141
- The Horus Heresy - Book Seven: Inferno by Alan Bligh (Forge World Series), pp. 25, 46, 50, 101, 108-111, 176, 179, 182-183
- Adeptus Titanicus - The Horus Heresy: Rulebook (Specialty Game), pp. 14-15, 58-59, 80-83
- Adeptus Titanicus - The Horus Heresy: Titandeath (Specialty Game), pp. 40-44, 95
- Adeptus Titanicus - The Horus Heresy: Traitor Legios (Specialty Game), pp. 28-33
- Titan Legions (2nd Edition), pg. 12
- False Gods (Novel) by Graham McNeill
- Flight of the Eisenstein (Novel) by James Swallow
- Galaxy In Flames (Novel) by Ben Counter
- Storm of Iron (Novel) by Graham McNeill
- Mechanicum (Novel) by Graham McNeill
- Vengeful Spirit (Novel) by Graham McNeill
- Cybernetica (Novella) by Rob Sanders
- Angel Exterminatus (Novel) by Graham McNeill, Ch. 27, pg. 416