Be'lakor

"'Let it be known that I, Bel'lakor, First Son of Chaos, shall forever be recognized as the one WHO STOLE SANGUINALA!'"Be'lakor is one of the first Daemon Princes in existence, and the first Daemon Prince of Chaos Undivided.

Official Canon
Be'lakor is one of the few characters that exists in both Warhammer 40,000 and Warhammer Fantasy, making his origins a bit ambiguous. In Warhammer Fantasy, he was the very first Chaos Warrior to ascend into becoming a Daemon Prince, and the only known prince of Chaos Undivided. However, he fell out of favor with the Chaos Gods when the forces of Chaos failed to stop the Elves from creating the Vortex and bringing their invasion to a halt; as punishment, he was forced into a more passive role of crowning the Everchosen, who would now lead Chaos' forces. Be'lakor has been seeking revenge ever since.

In Warhammer 40,000, he has similar issues with revenge and seeks out other daemons and champion of Chaos to overthrow, making him a useful wildcard in the Chaos Gods' power struggles. He's taken part in Abaddon the Despoiler's Black Crusades, though Abaddon knows that Be'lakor is simply trying to undermine him. No one in the Imperium is fully aware of Be'lakor's existence, and those that do assume that he's just a normal daemon.

TTS Canon
Be'lakor joined with the Black Legion and Chaos Cultists to lead an invasion on a world defended by Blood Angels, in an attempt to break the Blood Angels and put a permanent end to Sanguinala. However, he was stopped by Saint Celestine, who swiftly decapitated him with a single stroke and banished him back to the Warp.