Wyrms of the Ur-Tendril
The Wyrms of the Ur-Tendril are a Genestealer Cult.
The cult was entrenched amongst the Nordafrik under-archives on Terra when they were discovered by agents of the Ordo Xenos. Adeptus Custodes Captain-General Trajann Valoris refuses to allow the Deathwatch to send Kill-Teams, instead leading the purge in person at the front of a huge Custodes Shield Host. The Cult put up a brutal fight thanks to their sheer numbers, yet for every Custodian that falls hundreds upon hundreds of cultists are slaughtered. At last, Valoris himself beheaded the ruling Broodlord of the cult. Afterwards he ordered the creature's inner sanctum burned despite protests by the Ordo Xenos. Valoris refused to let anyone other than his comrades witness the foul murals that decorated the sanctum's wall, which portrayed a nest of fanged tendrils emerging from the heart of the Sol System to devour Terra whole.