Hive Fleet Ouroboris is a Tyranid Hive Fleet that may have been the first true Tyranids ever encountered by the Imperium of Man, with reports similar to what would be encountered thousands of Terran years later appearing as early as the 36th Millennium.
Hive Fleet Ouroboris is a nightmare from legend, a shadow from the stars that swoops from above on bat-like wings to devour its terrified prey. Death comes swiftly for those unfortunate enough to find themselves in the hive fleet's path, for Ouroboris strikes with blinding speed, cutting the heart from its victims before they even realise their doom.
There are some who believe that Hive Fleet Ouroboris was the very first Tyranid fleet to encounter Mankind. These theorists cite the ancient histories of Cardinal Miriamulus the Elder, who spoke of a nightmarish legion of "winged entities, aflame with infernal ague," which savaged the Helican Sector sometime in the early 36th Millennium. Relics hailing from that distant time bear distinctive bio-plasma scarring now recognised as consistent with Tyranid weaponry, and tales of flocks of winged horrors are certainly also consistent with Ouroboris' typical predatory behaviour. Among these relics was a later analysis Warlord-class Titan Mechanica Cranus, a veteran of the so-called Ouroboris Wars, which revealed distinctive bio-plasma scarring and pyro-acid burns that match current Tyranid Biomorphs. It is also believed that the Space Wolves possess trophies of that conflict, very similar to Tyranid lifeforms, such as the artifact the Fenrisians name the "Kraken's Egg."
It is now impossible to prove any direct connection, but recent Imperial encounters with the Hive Fleet designated Ouroboris have uncovered disturbing echoes of those old legends. Ouroboris favours massed aerial assaults, filling the air with so many Gargoyles and Harpies that those below must fight in near-dark. It strikes swiftly, honing in upon areas of strategic value with unerring accuracy. By slaughtering officers and destroying communications outposts, Ouroboris tears the eyes from its prey, leaving them confused and vulnerable. Yet perhaps the most disturbing aspect of the Hive Fleet is its strange, primordial biology. Encounters with Ouroboris have revealed that the organisms spawned by this Hive Fleet contain cruder, primitive versions of common Tyranid bio-weapons and organs. Unfortunately, these strange mutations have rendered many weapons and tactics designed to counter the Tyranid menace largely ineffective against the sky-swarms of Ouroboris.
Hive Fleet Ouroboris' primitive morphology bears an uncanny resemblance to the creatures recounted in a legend from the Imperium's dark past. Recent atrocities have only exacerbated its fell reputation.
- The Legion of Ouroboris (Unknown Date.M36) - In the 36th Millennium, Cardinal Miriamulus the Elder of Thracian Primaris records the history of the "Legion of Ouroboris," a vast host of winged xenos that descended upon the sector and stripped the life from dozens of planets. A grand Imperial crusade finally defeats these creatures in a twelve-day battle on the edge of the Eye of Terror.
- Shadow From The Stars (Date Unknown.M41) - Thousands of standard years after the death of Miriamulus, a chain of populated worlds bordering the Thracian Sector suddenly ceases communications with the wider Imperium. The Imperial fleets sent to investigate report back that every scrap of biomass on these planets has disappeared. Myths begin to spread on the remaining worlds of the sector, tales of a shadow from the stars that descends to devour the souls of the innocent.
- Ouroboris Rises (Date Unknown.M41) - A previously unrecorded Tyranid Hive Fleet invades the Thracian Sector. The Imperium designates this new threat Hive Fleet Ouroboris, a name taken from the ancient records.
- Fate of The Swarm Crushers (Unknown Date.M41) - A detachment of the Astra Militarum's Cadian 14th, known as the "Swarm Crushers" due to their storied exploits during the Second Tyrannic War, is sent to wipe out a tendril of Ouroboris that threatens the Desert World of Shukra. Drilled in the most efficient methods of slaying Tyranids, morale is high amongst the Cadians as they engage the first swarms. Confidence swiftly turns to panic as the Cadians' tactics have little or no impact upon the xenos. Pinpoint shots that should have ruptured vital organs have almost no effect, while salvoes of airburst shells filled with anti-Tyranid chemicals merely provide cover for the swarm. Stunned by the ineffectiveness of their defence, the Cadians nevertheless sell their lives dearly before they are devoured.
- The Sky Hunters (Unknown Date.M42) - The Silver Drakes, a newly-founded Primaris Space Marine Chapter, are tasked with turning back Hive Fleet Ouroboris. Their Inceptor Squads cut a swathe through the Hive Fleet's sky-swarms, hunting the immense winged monsters at their heart. Though the Drakes' tactics are effective, their numbers are few. Isolated and outnumbered, the Chapter digs in for a bloody running war.
The 4th Edition Codex: Tyranids states that Hive Fleet Ouroboros may have arrived in the galaxy within the Eye of Terror ahead of the other Tyranid Hive Fleets traversing the intergalactic void as the result of unstable currents in the Warp. However, the 5th Edition Codex: Tyranids altered the canon to explain that the Tyranids are not capable of using Warp travel, as they were in earlier sources.
- Codex: Tyranids (4th Edition), pp. 24-25
- Codex: Tyranids (5th Edition), pg. 19
- Codex: Tyranids (8th Edition), pg. 39