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 developed something of an arrogance complex, more concerned about his own glory than the Legion's. During the Drop Site Massacre, 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 double crossed him, tricking Tarvitz into killing Loyalist Space Marines before killing him.

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, there after 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.