Erebus

No single warrior did more to engineer the galaxy's ruin than Erebus, First Chaplain of the Word Bearers. A dark and subtle intelligence cloaked in the robes of a priest, he was among the earliest and most enthusiastic converts to the Primordial Truth, and where Lorgar sought damnation out of wounded faith, Erebus pursued it with the cold relish of a schemer who had found his purpose. He wielded ritual and lie with equal skill, and he moved through the Great Crusade sowing the seeds of betrayal with a patience that spanned decades.

It was Erebus who orchestrated the wounding of Horus upon Davin and arranged the false vision that seduced the Warmaster to Chaos, deliberately turning the Emperor's most beloved son into the instrument of rebellion. Every catastrophe that followed can be traced in some measure to his machinations, from the massacre at Isstvan to the summoning of daemons upon a hundred battlefields. He was despised even by many of his allies, who recognised in him a manipulator who served the dark gods above any brotherhood or cause, yet none could deny his terrible effectiveness. Arch-traitor among traitors, Erebus stands as the truest architect of the Horus Heresy.