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Mezoa is the most important and preeminent Forge World of the Adeptus Mechanicus within the Cyclops Cluster, a zone of space located in the galactic north of the Gothic Sector of the Segmentum Obscurus. To this day, Mezoa upholds strong ties with its inceptor, the great Forge World of Lucius, from whom it originated.

The Forge World's output (Tertius II Production Grade) is entirely dedicated to the production of first necessity war materiel such as arms and ammunition. In particular, Mezoa is responsible for the supply of macro-ordnance shells to the mustering point and fortress of the Imperial Navy at Port Maw, located in the neighboring Port Maw Sub-sector. This specialisation, as well as the important and well-equipped Taghmata maintained by the Mezoan Forge, have sometimes led the Forge World to be dubbed a "War World" by local Imperial Commanders, a reputation from which it still benefits.

Perhaps at odds with other domains of the Adeptus Mechanicus, the Forge World of Mezoa is distinctively militant and a firm supporter of the Imperial cause, most famously siding with the Loyalists during the dark events of the Horus Heresy when, isolated, cut-off and blockaded, the Forge World nevertheless succeeded in resisting enemy invasion during the Mezoan Campaign. These dark times would see it prevail both against the forces of the Warmaster Horus and those of its long-time rival Forge World M'Pandex, and remain defiant until its subsequent liberation.

More recently, in 139.M40, as an opening move during the 12th Black Crusade, more commonly known as the Gothic War, Mezoa came under attack by naval elements of Abaddon the Despoiler's forces, which seem to have been easily vanquished by the Forge World's defenders.


Ancient Departmento Cartigraphicae chart of the Cyclops Cluster at the time of the Horus Heresy.

An Adeptus Mechanicus stellar map depicting the Forge Worlds of the Segmentum Obscurus in the 41st Millennium, including the Forge World of Mezoa.

Mezoa is a vast, highly volcanic world which was naturally incapable of sustaining life but immensely rich in metallic elements. It was first colonised sometime in the Age of Strife by one of the three Explorator Arks sent out by the Forge World of Lucius after its quarrel with Mars, or according to other sources, to comply with an Edict Imperialis of the Emperor of Mankind and establish a further outpost to supply the armies of the Great Crusade.

What is known is that the orthodox Lucien Mechanicum used this opportunity to rid themselves of an aberrant sect growing in their midst whose schismatic ways would eventually give birth to the Cult of the Flame Eternal, which still dominates on Mezoa to this day. Thus, the Forge started as an isolated outpost of the Lucien Mechanicum and wasn't rediscovered during the Great Crusade until 540.M30. By then it had already expanded to something resembling a Forge World, a pocket-empire in the wilderness of space.

Mezoa itself was ringed with concentric layers of orbital docking platforms and habitation modules, the ash and cinder-wreathed surface of basalt having proven too unstable to support the original forge-fane's expansions. Beyond these rings lay outer boundaries of autonomous planetary defence batteries, kill-satellites and spatial mine-fields guarding the world's approaches. These formidable protections were far greater than those possessed by other individual Forge Worlds, yet they had not been constructed out of simple caution or paranoia, but had been tested on many occasions, particularly during the decades-long xenocide campaigns against the Mitu Conglomerate -- the Warp-capable alien pocket-empire which controlled much of the region of the Coronid Deeps before the coming of the Imperium.

Being besieged and bombarded on numerous occasions by the foul xenos before their eventual eradication by the forces of the Great Crusade, the Mechanicum of Mezoa had grown into one of the most militant and military-minded acolytes of the Machine God. This military prowess and battle-readiness vastly accelerated Mezoa's recognition as a fully-fledged Forge World, a status it obtained officially in 813.M30 under pressure of an Imperial Edict, for the Emperor of Mankind had wished for the Martian Mechanicum to establish a new outpost in the Cyclops Cluster to further consolidate Imperial power in that region of space.

Loathing to expend its might on a colony of one of its fiercest rivals, Mars delegated the task to reinforce and expand the colony of Mezoa to the authorities of Lucius. The Mezoan Forges were rebuilt under the direction not only of the Archimandrites of the Cult Mechanicus, but also the minds of both the Emperor and, it was said, the Primarch Ferrus Manus in later years after joining the Imperium. Great spindle-spires of metal rose out of the planet's surface, jutting like ziggurats, kilometres in height, from magma flows, and vast naturally-occurring lakes of molten iron dotted its landscape. Forge-fanes encircled the planet like a garland of barbwire.

This new Forge World was almost solely dedicated to the production of heavy arms and armour, its production ranging from Battle Cannon to warship hull plates; a fane of weapons and armaments solely intended to serve the needs of the ever-advancing front lines of the Great Crusade. Over the centuries leading to the Horus Heresy, the cult's dogma would be heavily influenced by the quixotic beliefs of the Hierarchs of House Hermetika -- who quickly became the Forge World's closest allies -- and would eventually give birth to the creed known as the Path of the Eternal Flame.

As militarily powerful as Mezoa had evolved to become during the years of the Great Crusade, one noteworthy idiosyncrasy of the Forge World was that it did not possess a sentinel Titan Legion, either of its own or as an ally. This was simply a consequence of the world's fluctuating and perilous terrain, which was entirely unsuited to Titan operations, both because of the planet's shifting topography and because where its scorched black surface was solid, such ground was likely to prove no more than a skin of solidified magma, treacherously unable to bear a Titan's weight.

Though this geological instability arguably robbed Mezoa of its most powerful potential guardians, it was also itself a final and telling line of defence, as events were later to prove. If the Mezoan forges somehow voluntarily renounced its Titan-production capacities, that was not the case for its Imperial Knight production-lines which worked tirelessly to equip those Imperial Knight Households oath-bound to it.

Most notable amongst these allies were the Knights of House Hermetika, which had stood at the side of the Forge World since the dark days of the Age of Strife, and noble House Vyronii. House Makabius nominally owed fealty to the lords of Mezoa, but later chose to side with the Warmaster and subsequently turned Traitor.

The Horus Heresy

By the outbreak of what was to become the Horus Heresy, Mezoa was a fully-operational Secundus I-grade Forge World whose impressive industrial output provided the forces of the Great Crusade with the arms and ammunition they would need to conquer the stars. Some of the models manufactured there, such as the Mezoa Pattern Meltaguns, Power Axes and Power Fists can still readily be found across the Imperium.

Most importantly, the resources of Mezoa and its vassal worlds aided in the installment and development of the Imperialis Armada facilities at Port Maw. The fleet stationed at these naval facilities depended on Mezoa's output for re-equipment, reparations and supplies, most notoriously in macro-ordnance shells and hull-plates which neither the neighbouring Mechanicus dominions of M'Pandex nor more distant Cyclothrathe could readily manufacture. This made Mezoa a highly valuable prize, a prize Horus would naturally seek to control as the Heresy unfolded.

The Emissary of Mars

At the dawn of the Horus Heresy, in 006.M31, both Mezoa and M'Pandex, the two greatest independent powers within the Cyclops Cluster, each received the Emissary of Mars; a Magos known as Regulus, or at least an entity bearing that name as there are extant contradictions in the record regarding the location of this individual at various times.

This senior Tech-priest of the Mechanicus had for many Terran years served alongside the Warmaster Horus, long before the betrayal at Istvaan, and was now part of Horus' inner circle and bound in allegiance to the Traitor's cause. Regulus came bearing dual patents of authority, the first from his Warmaster and the second from the hand of Kelbor-Hal, none other than the Fabricator-General of Mars, and by right the voice of the Omnissiah and Supreme Pontifex of the Mechanicum. Unknown to the Mechanicum authorities in the Cyclops Cluster, Kelbor-Hal had also thrown his lot with the Traitor Horus, sparking a civil war on Mars itself.

In stark contrast with the reception he would receive on M'Pandex, Regulus found Mezoa already geared for siege and conflict. Upon entering Mezoa's star system, his own craft, a swift Warp-runner, was stopped at the outer reach of the system and forbidden entry on the threat of immediate destruction. Mezoa had ever been a militant system, born of the ferocious wars of the Great Crusade and fashioned as much a fortress as a centre of macro-industry; its Magi fiercely independent from their kin elsewhere and just as fiercely loyal to the ideals of the Imperium whose expansion they served.

Here the influence of Mars or the authority of the Fabricator-General would count for nothing. Forced to communicate via only the most primitive two-way Vox-circuit with a hulking Mezoan battle-sphere rather than directly with the Forge World, and under the condition that any attempt at other communication or the passage of a data-djinn or other influence over the circuit would result in immediate annihilation, Regulus nevertheless put his case, invoking both the authorities of the Warmaster Horus and that of Kelbor-Hal to try to sway the rulers of the Forge World to his cause.

In response the Pentarchy of Archimandrites, which had by tradition ruled Mezoa, offered a blunt rejection of the Warmaster and a formal secession from the authority of Mars. Further, they called Regulus and Kelbor-Hal both apostate and blasphemers in the eyes of the Omnissiah, and pronounced them or any under their command to be under sentence of death if they entered the Mezoan System again. They finished their communication with a single word in judgement: "Heretek." Regulus fled.

The Blockade of Mezoa

Shortly after the denial of Regulus, a blockade of Mezoa was begun. With the help of the warships of M'Pandex -- both their own Mechanicum vessels and those they had since captured and now operated under a false flag, as well as more shadowed forces operating under Horus' direction -- the various star systems that fed into Mezoa were cut off from it, with the aim of choking off its supply of raw materials. Resources which, where possible, were now directed instead to the hungry forges of M'Pandex.

The Mechanicum on Mezoa actively resisted where they could, although they steadfastly refused to sally forth with their full force, treating the attacks as bait in a trap. Instead, they ordered their outposts and detached forces to resist until destroyed, and on far-flung mining stations and on the decks of mass transporter vessels, the forces of the two Forge Worlds, long rivals but never before enemies, clashed in mortal combat.

Mercy was not to be found in either sides' circuit-augmented hearts, and the Adsecularis cohorts of M'Pandex were matched savagely by the armoured centuries of Mezoa's Thallaxii which, while fewer in number, vastly outmatched the Adsecularis in individual might. In what seemed now to outsiders to be a civil conflict within the Mechanicum rather than a move in a wider game, refinery stations on Jujya and Gunnar's Rock burned, and on New Providence, thousands of fleeing vapour-mill workers caught between the battle lines, were crushed beneath the tracks of Krios tanks and slaughtered needlessly in the withering crossfire of blazing energy weapons as both sides utterly ignored them.

By the end of 007.M31, the Mezoan System had been almost entirely cut off from the wider Imperium, and now enemy warships were circling the edge of its outer reaches as sharks waiting for prey. Mezoa was a prize not to be underestimated, but it was also fortified beyond ready conquest, even by a force as large as a fully marshalled Expeditionary Fleet. However, once cut off from aid and from giving aid to others, it was now effectively neutralised. As other star systems declared for Horus, the blockade was increasingly reinforced, most decisively after the Fall of Port Maw which saw the blockading fleet reinforced by turncoat siege-monitors of the Armada Imperialis, the pale-hulled Legion warships of the Death Guard Legion, as well as dark-hulled vessels of unknown provenance that now bolstered the Traitors' forces.

Eventually the force amassed was sufficient to mount an attack on the beleaguered Forge World. This first great assault would be hurled back under mysterious circumstances, the Mezoan Magi having somehow succeeded in turning the very fabric of their molten world against their attackers, causing vast swathes of the planet's mantle to suddenly and violently collapse under the enemy's landing zones. The wilder reports even told of colossal island-size chunks of magma hurled up into space and flung at the attacking ships.

Regardless of the truth, that the Traitor forces had been hurled back was known, but Mezoa was but a light in a sea of night, and how long it would last could not be predicted. The initial blockade would become a full-fledged siege, marking the beginning of the Siege of Mezoa which would continue for the better part of the Horus Heresy. The Siege of Mezoa would mark one of the few confirmed appearances of Traitor elements drawn from the remains of the three Loyalist Legions massacred at Istvaan V, the Forge World's siege seemingly having mobilised a company-size element of Blackshields wielding both the weapons of the Iron Hands and the Sons of Horus.   

The Sieges of Mezoa

While in the first years of the Horus Heresy, Mezoa had already repelled two attacks on its soil, the decision to end Mezoa's defiance was quick to come. Having long-departed this sector of space, Horus himself asked his followers to finally descend upon the Forge World and raze it. Planning for a renewed push towards the Sol System, Horus knew full well that he could not afford to leave any weaknesses at his rear if he was to conquer the throne of his father.

In fact, the order was so widely promulgated that several illuminated copies of the manuscript still exist to this day in the great Imperial Archive of Terra. Horus' request was carried to the great Armada Imperialis stronghold of Port Maw in the Manachean Commonwealth, which had fallen early on to the Warmaster during the famed Treachery at Port Maw incident. Here, those fleets loyal to Horus were resupplied and repaired, reading themselves for the next stage of the galactic civil war.   

However strategically important, the destruction of Mezoa would be no easy task. Every Traitor commander knew that if victory was to be achieved at all, it would undoubtedly be bought at a high cost. Few commanders present at Port Maw were ready to let their forces be badly mauled so early in the campaign to overthrow the Imperium, yet the Warmaster's will had to be done, and so a quiet war of influence, lies and veiled truths soon gripped Port Maw to designate one amongst their number to pick up this poisoned chalice.

This political bickering often ended in bloodshed as Astartes are first and foremost crafted for war and armed combat, until the proverbial scapegoat presented itself in the form of Autilon Skorr of the Alpha Legion. Lacking both the presence of their respective Primarchs and that of an influential patron at Port Maw and still humbled by their defeat at Stranivar, it soon fell to the 78th Chapter of the Alpha Legion and the ill-fated 114th Grand Battalion of the Iron Warriors under Nârik Dreygur to prosecute the Mezoan Campaign.

Benefitting from his rank as Consul-Delegatus of the XX Legion, Skorr was designated as overall commander of the besieging Traitor forces and used his newly acquired status to appropriate the lion's share of the available resources at Port Maw to bolster and reequip his own companies, leaving the Iron Warriors of the 114th Grand Battalion little chance to recover from their losses at Stranivar.

With Perturabo and the bulk of the IV Legion far distant, the remaining Traitor officers did not feel compelled to aid Dreygur, who they largely perceived as a failure, and thus the Iron Warriors had no choice than to make what repairs they could and embark on this new mission. Autilon Skorr had not given up his ambitions and his tenacity was rewarded with one of the largest autonomous commands assigned to a simple officer, the Mezoan taskforce compromising no less than 9,000 Space Marines, three full Taghmata heralding from the Forge World of M'Pandex and more than 20,000 human troops.

His fleet numbered a dozen capital vessels, amongst which two were of the fearsome Mangonel-class Bombardment Cruisers. Apart from the Primarchs themselves, none had commanded more men or more warships, but what truly spurred on Skorr was the prospect of victory, for if he won he knew that he would benefit greatly from Horus' favour. Assured by his Davinite priests of Chaos that the passage of the fleet would be swift and sure, the invaders made way for Mezoa.

During the orbital battle that marked the opening of the Third Siege of Mezoa, the Loyalist defenders received help from an unexpected quarter: the Ebon Drake, a Strike Cruiser of the Salamanders Legion. Daring the guns of the enemy, the Ebon Drake ran the gauntlet in order to allow its complement of Astartes warriors to join the battle on the ground.

Under the guidance of their illustrious leader, Cassian Vaughn, the Dracos Revenant, this company of warriors -- which had taken to calling themselves the Disciples of the Flames -- would prove instrumental in the defence of Mezoa's forge-fanes and in repelling the invaders. In particular, Cassian Vaughn's actions would lead Nârik Dreygur and the survivors of the 114th Grand Battalion of the Iron Warriors to renounce their oaths to the Traitor Horus and join the Loyalist camp.

At Present

Mezoa remains to this day a hive of industrial output and military might, growing past its former borders as the Imperium called upon it to equip its armies in the post-Heresy era. The Forge World still continues its oath-bound task to supply the vast installation of Port Maw and the battle fleets stationed there, having contributed lately by adding a significant weapons-system to the Imperial Navy's arsenal: the Mezoa Pattern Hybrid Lance, which sacrifices range for greater penetration capabilities.

Since the 38th Millennium, Mezoa has even added to its defenders. The proud Astra Militarum regiments of the Lambdan Lions fight alongside the Mechanicum and hail from the two moons of Mezoa; the regiment's Schola Progenium having brought forth such elite formations as the 133rd Lambdan Lions Tempestus Scions, and the resources of the Forge World making them one of the best-equipped formations in the Imperium.   

Mezoa Appearance

Mezoa Iconography

Both the Magi of Mezoa and the Hierarchs of House Hermetika have appended flame iconographies rather than the more traditional symbols of the Machine Cult. It is, however, unclear if this symbolism is a silent reminder of their volcanic homeworld or somehow connotes a deeper spiritual meaning for the Hierarchs.


  • Codex: Adeptus Mechanicus (8th Edition), "Bastions of the Machine God," pg. 12
  • Codex: Militarum Tempestus (6th Edition), pg. 17
  • Imperial Armour Volume One - Imperial Guard & Imperial Navy, pg. 8
  • The Horus Heresy - Book Four: Conquest (Forge World Series) by Alan Bligh, pp. 24-27, 35, 54, 66, 98
  • The Horus Heresy - Book Six: Retribution (Forge World Series) by Alan Bligh, pp. 33-53