Tsara'noga

Tsara'noga, the Outsider, is the C'tan whose gaze made witnesses prefer suicide to company. Eldar myth says Cegorach tricked it into eating its own lesser kin until the swallowed fragments cut it from the inside and the star-god folded in on itself, screaming.

Some accounts put the mad remainder in a Dyson prison beyond the galactic south, still intact while its siblings were shattered into shards. Inquisitors have even wondered, quietly, whether Tyranid hunger is a long echo of that exile. Farseer Lsathranil's old prophecy still expects the Outsider to come back for a harvest when the galaxy is already red.

The Nightbringer is death. The Deceiver is a lie. The Outsider is the neighbour you do not check on.