Lucius

Lucius the Eternal is the sickest fuck in the Emperor's Children, an entire Legion of sick fucks. That's really saying something.

Back during the Great Crusade, Lucius was Captain of the 13th Company, where he was the best swordsman in the Legion. Meaning, in an entire army of perfectionists, no one was better at sword fighting that Lucius. He was also extremely arrogant, more concerned about his own glory than the Legion's. During the Battle of Isstvan III, Lucius initially sided with the Loyalists against the Chaos Space Marines. But when command of the Loyalist Emperor's Children was taken by Saul Tarvitz, Lucius got jealous and betrayed their stronghold in exchange for joining the Traitors, tricking Solomon Demeter into killing Loyalist Space Marines before killing him. Proving that the age old adage "It is better to die for the Emperor than live for yourself" may have some truth to it.

Despicable, but it's after the Horus Heresy where things start to get creepy. During one of the random gladiatorial games the Emperor's Children partake, Lucius was struck down. The experience was so enjoyable that it caught Slaanesh's notice. Not wanting to lose so devoted a servant, Lucius was resurrected in his killer's own body. After that, anyone who struck down Lucius and felt any satisfaction from the act would find themselves possessed by Lucius, who thus became known as Lucius the Eternal.

In between Black Crusades, Lucius is said to wander that Eye of Terror, supposedly looking for either the location of Fulgrim's pleasure world, or to find a means to weaponize his penis.