Lucius

Lucius the Eternal is the sickest fuck (you must be new here... he's been driven out of the legion a while ago) 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 than 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. Feel satisfaction? Well then let's kill him with a servitor and a bunch of 'nids! May have in fact strengthened the Imperium by getting killed by more skilled Chaos Champions and Xenos.

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.

In actual gameplay, Lucius is a special character that mostly focus on killing infantry, with wargear and abilities that don't do much against anything else (his coming back from the dead rule isn't used in gameplay). While he's good at what he does, he's usually overshadowed by Kharn and Abaddon in the melee HQ area since they are effective in melee against most anything.