The Legio Osedax ("The Cockatrices") is a Loyalist Titan Legion of the Collegia Titanica, one of the military arms of the Adeptus Mechanicus. It hails from the Forge World of Phaeton, located in the Segmentum Solar.
There is very little information in Imperial records about this obscure Titan Legion, and it is unknown when they were founded. However, it is likely that this Titan Legion's creation occurred some time during the Great Crusade following the transplant of several Phaetonite magi to the world of Atar-Median.
When they departed their homeworld, the Phaetonites took with them a core of ancient god-engines that had once defended Phaeton in the anarchy of Old Night.
These Titans went on to become a part of the newly-founded Legio Atarus, while those Titans left behind on Phaeton became the core of what was called the Legio Osedax.
Long had Phaeton, the homeworld of the Legio Osedax, held a feud with Mars and when the Fabricator-General Kelbor-Hal revealed his treachery during the Schism of Mars the Cockatrices fell prey to his wrath, reduced to nearly a quarter of their strength by deadly ambushes and nefarious machine curses unleashed by the Dark Mechanicum.
Those that remained were reborn in the fires of war, though their strength was fated never to return to the glory days of the Great Crusade.
- War in the Shedim Drifts (ca. 800s.M30) - This was an Imperial Compliance action carried out against the 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 , also called the "Firebrands," was cruelly used, and without their foreknowledge, they were sent to probe the lines of the Aeldari, and were subsequently destroyed. Heavily engaged and outnumbered by the xenos machines, the promised support to the Legio Atarus by the Legio Mortis did not arrive while the Titans of the Legio Fureans, holding the Firebrands' flank, crumbled. A full demi-legion of the Firebrands was encircled and all but destroyed, with only a single Warhound Scout Titan surviving the engagement. In the aftermath it was revealed that the attack had been a ruse, a distraction planned by Horus to divert the enemy away from the direct defence of their craftworld. The plan worked, and the Luna Wolves destroyed Mór-ríoh'i in a surprise assault. The grievous losses suffered by the Firebrands due to the Legio Mortis and the Legio Fureans' refusal to fight effectively alongside them due to internal Mechanicum politics, was a grievous injury not soon forgotten.
- Battle of Beta-Garmon, "The Titandeath" (006-013.M31) - The Legio Osedax took part in the long and savage Beta-Garmon campaign, which included hundreds of war zones across dozens of worlds. Even before the greater armies of the Warmaster Horus reached the star cluster, the battle lines had long since been drawn, and fighting had been going on for many Terran years. It was a cauldron of battle that would consume millions of lives before its end and see the demise of entire Titan Legions, earning this campaign the dire moniker of the "Titandeath." The Beta-Garmon Cluster, also known as the gateway to the Throneworld of Terra, was the last hurdle that the Warmaster Horus' forces had to overcome before they reached the Imperium's capital world. Heavily fortified by the Loyalists, Beta-Garmon would become one of the greatest and bloodiest battles of the Horus Heresy, as well as one of the longest-lasting.
- The Balthor Sigma Intervention (012.M31) - A blood-maddened pursuit force of Traitor World Eaters engaged in hunter-killer operations against defeated Loyalist forces was counterattacked by a Legio Osedax demi-legion at Balthor Sigma. The World Eaters were supported by numerous super-heavy tanks and therefore able to hold the Cockitrices' Titans at bay until, in quick succession, the tanks were engaged from an unexpected quarter and destroyed in short order. Entirely unheralded, a force of xenos Titan analogues, later determined to belong to the Aeldari Fir Iolarion clan of Craftworld Lugganath, intervened to devastating effect. It is only when Traitor-aligned Titans of the Legio Fureans fought their way through to reinforce the Traitors' lines that total defeat was averted. At the conclusion of the battle, the xenos Titans disengaged and vanished into the ash-shrouded depths of Balthor Sigma's equatorial magma wastes.
- The Titandeath (356.013.M31) - More than twenty Titan Legions (including the Legio Osedax) and hundreds of Titans converge on Beta-Garmon II as the Loyalists seeks to strike a decisive blow to reclaim Nycron City and clear the system of Traitors. Shoulder to shoulder, the Loyalists meet their enemies in battle like infantry formations facing off, only on a much grander scale, Knight banners racing around their feet like hunting hounds. Many Titan Legions are extinguished forever in the day of battle that follows, and even legendary formations such as the Legio Astorum and the Legio Fureans are reduced to a fraction of their initial size. By the time the Loyalists reach the shattered walls of Nycron, both sides are staggering over fields of fallen god-machines, while the light from Beta-Garmon II's burning hive cities is visible in space.
- Dark Marches Crusade (610.M36) - A demi-legion of the Cockatrices contributed to the Dark Marches Crusade, launched in the wake of the Age of Apostasy, to retake the region of the Segmentum Tempestus then called the Dark Marches. With the blessing of Terra, Segmentum authorities launched a number of Crusades and campaigns to destroy the lingering threats within their domain, and push back and fortify the borders of the Imperium itself. The most successful of these was the Dark Marches Crusade, commanded by Lord Tempestus Hal Orpheus, lineal descendant of the Rogue Trader House that had first explored the region's sinister reaches millennia previously. Over 600 worlds were claimed in the name of the Emperor and many new Imperial sectors were established, perhaps most famously the Orpheus Sector.
- Battle of Herald's Fall (Unknown Date) - During the Battle of Herald's Fall, four household detachments of House Krast's Imperial Knights fought the invading Ork forces of WAAAGH! Spleenrippa alongside the Legio Osedax. Though they lost fully half of their number over the course of the fighting, the three surviving Knights earned what passes for acclaim from the Fabricator-General of Mars himself after bringing down Spleenrippa's Great Gargant in a combined assault before it could smash apart the irreplaceable war machine called the Ordinatus Mars. In the wake of the battle, Headtaker Forillus, the Knight who destroyed the head of the Warlord's Gargant with his Thermal Cannon, killing Spleenrippa, was seconded to the Cockatrices.
- Terra's Calling (Imperator-class Titan) - The Terra's Calling was an Imperator-class Titan of the Legio Osedax that fought in the Third Battle of Nyrcon during the Beta-Garmon campaign, better known as the "Titandeath."
- Traves - Traves was the grand master of the Legion during the Balthor Sigma Intervention in the Horus Heresy.
The Legion colours of the Cockatrices are a quartered colour scheme of aquamarine blue and white with brass trim.
The Legion's alternate scheme is sometimes displayed as white with blue checkers.
The Legion badge of the Cockatrices is a cockatrice facing leftwards with its wings outstretched, imposed over an orange circle outlined in black. The field behind it is quartered in tan and dark blue.
- Adeptus Titanicus - The Horus Heresy: Rulebook (Specialty Game), pg. 17
- Adeptus Titanicus - The Horus Heresy: The Defence of Ryza (Specialty Game), pg. 32
- Imperial Armour Volume One - Second Edition: Imperial Guard, pg. 10
- Imperial Armour Volume Twelve - The Fall of Orpheus, pg. 18
- The Horus Heresy - Book Two: Massacre (Forge World Series) by Alan Bligh, pg. 157
- The Imperial Knights Companion (6th Edition), pg. 95
- White Dwarf 110 (UK), "Adeptus Titanicus: Eldar," by Jervis Johnson & Graeme Davis, pg. 67
- White Dwarf 108 (UK), "'Eavy Metal: Adeptus Titanicus Titans & Banners," pg. 76
- Adeptus Titanicus: Dominus (PC Game)
- Titandeath (Novel) by Guy Haley
- Warhammer Community - The Defence of Ryza: Your First Look (31 Mar 2020)