Hive Fleet Ouroboris
Not to be confused with the Ouroboros, an entity captured by the Dark Angels that is certainly neither a Tyranid nor a heretical entity.
Tyranids during the Great Crusade?
The time of arrival of Hive Fleet Ouroboris is an interesting point of discussion. The earliest mentions of this Hive Fleet were made in M36, when the then-Cardinal of Thracian Primaris, Miriamulus the Elder, wrote in his life work about how, in an earlier age, the Helican Sector had been attacked by what he called the "Legion of Ouroboris". He mentions how this attackers could strip all the life wherever they went, and how "winged monsters vomited from the bellies of bigger beings" composed most of its forces. In this documents, it is mentioned that the Emperor himself had to fight them to drive them back, culminating in a two weeks long battle over a Warp Rift close to the Eye of Terror.
This description made certain Imperial scientists think years later that this was humanity's first encounter with the Tyranid menace, although this is a point of contention. True, the descriptions written here are close to Tyranid biology, strategy and behaviour (and the idea of the Emperor personally fighting Tyranids is an amazing concept) we cannot fully prove this was the case. By the time of the Great Crusade, there were hundreds, if not thousands, of alien species Mankind had to deal with and there were also some species of insectoid nature that could have fit the description. Hell, this was close to the Eye of Terror, and if the Emperor part was true, maybe it was an early daemon encounter that was censored later as a xenos attack. That would certainly match the "strip all life" part (Exterminatus on the planets before the rumours of daemons spread throughout the galaxy, then blame the blasted xenos). There's also the problem of its location: they appeared very close to the Eye of Terror, and not in the fringes of the galaxy. Most experts believed this proved the "Legion of Ouroboris" couldn't be a Tyranid fleet, but the arrival of Hive Fleet Leviathan in M41 by moving down the galactic plane meant that this argument is now invalid. To be honest, the fact that most of the early chronicles about Ouroboris were made millenia after its encounter make these story tacky at best. Still, if this "Legion of Ouroboris" was indeed a Tyranid Hive Fleet, it would mean this was Mankind's first conflict with the Tyranids; Hive Fleet Tiamet's discovery was a millennia before them, but they were content to stay in the Tiamet system instead of trying to OM NOM NOM the galaxy, so it's debatable whether they count as a proper contact. The immediate answer to this puzzle might be warp time chicanery, but tyranids don't use a warp based FTL so baring somebody in the future throwing a hive fleet into a warp storm that possibility seems unlikely.
Also, some people believe that the tyranid-like creatures living in Fenris are, in fact, Tyranid bioforms captured and sent to that planet by the Space Wolves captured during this campaign as war trophies many millenia ago, but this is also not confirmed.
Confirmed sightings of Hive Fleet Ouroboris
The first "modern" sighting of Hive Fleet Ouroboris was in M41, when it attacked the Thracian Sector. Named after the legends of the "Legion of Ouroboris" (furthering increasing the confusion between the old legends and the current threat), several campaigns have been launched against this alien threat. Ouroboris favours massive aerial swarms of Gargoyles and other flying bioforms, in a manner alarmingly similar to its namesake. Especially worrying is that most chemical anti-Tyranid weaponry doesn't seem to faze them in the slightest. Also, some experts have discovered that the DNA of the fleet contains older models of creatures and genetic structures in comparison to other more recent Hive Fleets, which similarly suggests that the Tyranids may indeed have been in the galaxy longer than anyone believed. Currently a chapter of Primaris Space Marines, the Silver Drakes, is keeping this threat in check although this Astartes chapter is currently undermanned and with very few reinforcements, so the war is either going to last a very long time, or not long at all.
On a meta narrative note if Hive Fleet Ouroboris was sent back in time and eventually became the Legion of Ouroboris then Game Workshop basically gave that twist away with the name. An Ouroboris is a snake that eats it's own tail, no beginning, no end and it is a symbol of infinity, of endlessness. So if Hive Fleet Ouroboris ended up in a stable time loop, then it's name would be perfect for it's ultimate fate.
|Tyranid Hive Fleets|
|Major Hive Fleets:||Hive Fleet Behemoth - Hive Fleet Kraken - Hive Fleet Leviathan|
|Minor Hive Fleets:||Hive Fleet Colossus - Hive Fleet Dagon - Hive Fleet Gorgon - Hive Fleet Hydra - Hive Fleet Jormungandr |
Hive Fleet Kronos - Hive Fleet Moloch - Hive Fleet Naga - Hive Fleet Ouroboris - Hive Fleet Tiamet