The Deceiver

"'State of affairs is actually what amounts to a collection of rocks desperately clawing their inanimate ears out as the organic shriek-machines are pulling them all STRAIGHT down the End Times pit.'"Mephet'ran, The Deceiver is one of the C'tan. He is currently the most active of the C'tan, actively plotting behind the shadows.

Official Canon
The Deceiver 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
Ecclesiarch Decius XXIII calls the Deceiver 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.

The Deceiver appears in Commoragh, using Tahril as his pawn within the Kabal of the Poisoned Tongue. The Deceiver ranted and raved about the End Times, and was adamant that Lady Malys's forces would be needed to prevent Kaldor Draigo from re-entering realspace, considering him to be a major threat to the galaxy at large.

During Malys's travel to dethrone Vect, she informs Tahril (and the Deceiver) that Vect is torturing the The Fyodperor, surprising the Deceiver. After learning this and the fact Malys wants to rescue the Imperials at the Arena, he reveals Tahril the reason why he wants to kill Kaldor Draigo: he's the result of a joint experient between him and the Emperor to fuse Draigo with a fragment of the Outsider, but end up being more powerful than he planned, forcing his hand to orchestrate the events behind Draigo ending in the Warp. With Draigo now in Commorragh, and Malys's plan of rescuying the Imperials, the Deceiver wants Malys to kill Draigo because only a god can kill Draigo, and Malys is been possessed by Cegorach. He also wants to kill the Fyodperor. After this, he talks with another fragment of himself who's also in Commorragh, ordering him to do anything to ensure Draigo's death.

It is also revealed that at least two fragments of the Deceiver lie on Mars, with one being under Belisarius Cawl's possession.

Trivia

 * The Deceiver is one of the few characters in TTS who have multiple expressions, notably, he has a scowling face and a screaming face (most other characters either have no expressions or have their faces slightly morphed to convey different emotions).
 * His Screaming face resembles Video Game developer Todd Howard. It is unknown whether this is a purposeful joke regarding Todd's reputation of lying or a simple coincidence.
 * Unsettlingly, when first emerging from his weird cube form thing, the Deceiver makes the exact same sequence of coughs and vocal emanations that the Emperor did when his text-to-speech device first started up at the beginning of the series. What importance this has is yet to be revealed.