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 founding 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, they 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 the Legio Osedax.
- Lament of Shedim (Unknown Date.M31) - This was an Imperial Compliance action carried out against the Eldar 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 Eldar, 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.
- Dark Marches Crusade (610.M36) - A demi-Legio 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 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 survivors above 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 gutted the Warlord's Gargant with his Reaper Chainsword, was seconded to the Legion.
- Battle of Balthor Sigma (Unknown Date) - The Cockatrices Titan Legion battled alongside the Eldar Titans of the Fir Iolarion clan of the Craftworld Lugganath against the Legio Fureans ("Tiger Eyes") Traitor Titan Legion.
None listed in current Imperial records.
- Traves - Traves was the Grand Master of the Legion during the Battle of Balthor Sigma.
The colours of the Cockatrices are not listed in current Imperial records.
The Legion badge of the Cockatrices is a cockatrice facing leftwards with its wings outstreched, imposed over an orange circle outlined in black. The field behind it is quartered in tan and dark blue.
- 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", pg. 67
- White Dwarf 108 (UK), "'Eavy Metal: Adeptus Titanicus Titans & Banners," pg. 76