In the days just before the birth of the Great Rift in the late 41st Millennium, a Genestealer Cult calling themselves the Wyrms of the Ur-tendril were discovered by Ordo Xenos agents, entrenched amongst the Nordafrik under-archives on Terra.
Adeptus Custodes Captain-General Trajann Valoris refused a request by the Deathwatch to send Kill-teams against this threat, instead leading the purge in person at the head of a huge Adeptus Custodes Shield Host.
The cult put up a brutal fight, their sheer numbers and fanaticism allowing them to drag down one Custodian after another and tear them limb from limb. Yet for every one of the Custodians that fell, hundreds upon hundreds of malformed cultists and Aberrants were slaughtered.
At last, Valoris himself beheaded the monstrous Broodlord that ruled over the cult. He ordered the creature's disturbing inner sanctum burned despite the protests from the Ordo Xenos investigators -- Valoris refused to let anyone other than his comrades witness the foul mural that decorated the sanctum's back wall, of a nest of fanged tendrils emerging from the heart of Sol itself to devour Terra whole...
- Codex: Adeptus Custodes (8th Edition), "The Tale of the Ten Thousand", pg. 33