Be'lakor

Be'lakor is the first Daemon Prince, the Dark Master who was never content to be any one god's favourite and has spent eternity trying to become the one they all have to tolerate. He crowned himself in shadows before the Heresy had a name, and he still resents every rival who got a pantheon of their own.

He has possessed mortals, stolen knights, and walked the Era Indomitus as a power who will sabotage Vashtorr as gladly as he will sabotage the Imperium. The Disciples of Be'lakor are not a Legion. They are a grudge with an army.

The four Gods used him. He has not forgiven that, which is as close as the Warp comes to a personality.