Mephet'ran, The Deceiver is one of the C'tan.
Official Canon Edit
The Deciever is foremost among the C'Tan, playing an active role in the War in Heaven up to modern day. He tricked the other C'Tan into eating each other after they had defeated the Old Ones, and while he may have been split into shards like the others, that hasn't stopped him from continuing to manipulate the galaxy. He is a shapeshifter who uses various disguises to manipulate others into doing his bidding; for a time he was an Imperial governor who survived assassination by absorbing the phase blade used on him. Another time he helped Abaddon the Despoiler to obtain the Blackstone Fortresses in his twelfth Black Crusade. His interest seems to be to bleed the Imperium and Chaos dry to give the Necrons less resistance.
TTS Canon Edit
Ecclesiarch Decius XXIII calls the Deciever a "straight-up idiot" for undermining his allies during the War in Heaven for seemingly no reason. The Emperor also complained about "that punk-ass C'Tan God" trying to join his game of Paradox-Billiards-Vostroyan-Roulette-Fourth-Dimensional-Hypercube-Chess-Strip Poker.