Ouroboros

For the D&D deity that carries the motive of the snake biting its own tail, see Jazirian.

Not to be mistaken with Hive Fleet Ouroboros

The Ouroboros is a warp creature buried underneath Caliban. It resembles a giant worm with huge pincers and a human face. While it does not consume itself, it is thought that it may have played a role in making the Dark Angels turn against each other and might have also been responsible for the planet's infestation of monsters. For some reason, despite the assumption that sorcery was extra outlawed on Terra, a group of Terran sorcerers both knew what it was and were trying to get rid of it. Oh, and it is a component of the Old One construct that built the Webway (along with the Tuchulcha, a sentient Warp engine that has aligned itself with the Dark Angels since before the Horus Heresy for its own reasons, and the Plagueheart, a planet that looks like the galaxy's largest Great Unclean One).

It has since been revealed to be a piece of the Key, a relic that allows its user to open a portal to just about anywhere in the galaxy.