Lucius the Eternal

"'Hey there gal pals, it's yo boy Lucius, coming at you with another BOMB-ASS PRANK!'"Lucius the Eternal is a member of the Emperor's Children, and a Champion of Slaanesh. Slaanesh has granted him a special form of immortality where if the person who slays him feels any satisfaction over the kill, that person will become the new Lucius.

Official Canon
During the Great Crusade, Lucius was among the Emperor's Children's best swordsman. During the battle of Istvaan III, he had initially been part of the loyalist forces, but was quickly turned to Chaos partly by jealousy and partly by hearing the music of his traitor brethren and betrayed his commander.

During the Horus Heresy, Lucius was killed by one of the Raven Guard's commanders, but mysteriously rose back up from the dead. But it was after the Heresy that Slaanesh had chosen Lucius to be its champion, and anyone who slew him would become Lucius.

TTS Canon
Lucius is even more demented and terrifying than official accounts depict. He has a falsetto, Mickey-Mouse type voice and takes great pleasure in torturing people in what he refers to as pranks. He even has his own show that he posts to Spacebook called "Lucious Lucius Prankz." Lucius attempted to create a new holiday called Slaaneshmas, but was unable to spread the holiday cheer until he teamed up with Ahriman and Fabius Bile.

Trivia

 * Lucius’ first scene is a reference to a clip from Cryaotic’s playthrough of Rust.