Erebus

What a bastard.

Erebus was a Dark Apostle of the Word Bearers and one of Lorgar's trusted lieutenants. During a certain time, he become a devout servant of Chaos and co-conspired 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. He has shown notable distaste for Abaddon's status as Warmaster of Chaos, believing that a new Warmaster is needed. Which goes to show that just because you've spent ten thousand years chilling in the Eye of Terror doesn't mean that you forfeit common sense.

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.