The Behemoid Undercult is a Genestealer Cult that operates on the fringes of the Realm of Ultramar. This hidden organisation is of such cunning it has infested several Ultramaran worlds, despite continued attacks from the Ultramarines Tyrannic War Veterans trained by Chaplain Ortan Cassius.
There are rumours the cult's founders worshipped a battle-scarred Tyranid monstrosity of Hive Fleet Behemoth trapped in ice on the world of Calth after the First Tyrannic War, and that they still revere that great beast alongside their own Genestealer Patriarch as a saint of their new order.
That creature, a unique Tyranid Carnifex known as "Old One Eye" in the spacefarer tales that surround it, is of such totemic importance to the Undercult that they ritually scar and tattoo themselves, or even cut out their own right eyes, in homage to the creature, seeing it as a prophet of the xenos god they simply call "Behemoth."
- Codex: Genestealer Cults (8th Edition), pg. 32
- Codex: Genestealer Cults (7th Edition), pg. 50