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 does cause the Dark Angels to turn against each other. 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 an engine that acts more like a Great Unclean One and some other thing).