In M36 the then-Cardinal of Thracian Primaris, Miriamulus the Elder, recorded the history of the "Legion of Ouroboris" that attacked the Helican Sector in an earlier age. The legion was described as consisting of winged monsters, vomited from the bellies of great beasts, that descended from the heavens and stripped the countryside of life. The Emperor himself is attributed with leading a crusade that resulted in the monsters turning on their own forces, culminating in a twelve day long battle over a Warp Rift close to the Eye of Terror.
A later analysis of the Warlord Titan Mechanica Cranus, a cited veteran of the Ouroboris Wars, revealed distinctive bio-plasma scarring and pyro-acid burns that match Tyranid bio-weaponry. It is also believed that the Space Wolves have trophies, very similar to Tyranid lifeforms, dating from this epoch, such as the so-called Kraken's Egg. Augur-dating techniques date the Titan's damage as being caused post-Heresy, making the Emperor's supposed presence in the Ouroboris Wars extremely unlikely.
- Codex: Tyranids (4th Edition) states that Hive Fleet Ouroboros may have been thrown into the Galaxy and deposited into the Eye of Terror ahead of the main Tyranid Hive Fleets due to treacherous Warp currents.
- However, Codex: Tyranids (5th Edition) has since stated that the Tyranids are not capable of using warp travel, as they were in earlier sources.