Erebus

Erebus was a Chaplain of the Word Bearers and one of Lorgar's trusted lieutenants. During a certain time, he become a devout servant of Chaos and co-conspirated with Kor Phaeron in order to bring the downfall of the Imperium. Though, to Erebus' credit, whilst Kor Phaeron was able to corrupt his Primarch, it was he who was indirectly responsible for the Horus Heresy. For it was he who had corrupted Horus, and he, along with Horus, masterminded the drop site massacre of Isstvan III. He continued to serve Horus as his spiritual advisor until his defeat in the hands of the Emperor. Erebus currently holds a position in Lorgar's Dark Council.

His name is based on the Greek entity Erebus, who was the son of Chaos. And it quite suits him since he "Is" a Chaos Space Marine.