Lucius

The sick bastard in all his sick glory.

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 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.

During the rest of the Heresy Lucius put his skills to good use murderizing loyalists (and organising an exorcism / BDSM session for Fulgrim). He continued being an arrogant supremely skilled bastard until he met Nykona "I don't use the floor" Sharrowkyn, a Raven Guard badass who proceeded to make Lucius his bitch on two separate occasions before stabbing him through both hearts and killing him..

Except the last part didn't quite stick, because he mysteriously got up again after being very definitely dead. Not even Fabius knows how he pulled this off.

The Iron Hands found a way to make his Come Back From The Death Ability useless

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. To all the MANLY FOLLOWERS OF Khorne, this makes Lucius a total failure compared to Kharn, as Kharn has been killing stuff for just as long as Lucius without dying once. To all the FOOLISH IMPERIAL SCUM, this is the most terrifying thing ever: they know they'll never get him with an exterminatus or orbital bombardment because of plot, and they know that their greatest champions (who have their own plot armor) can't beat him either. May have in fact aided the Imperium by getting killed by more skilled Chaos Champions and Xenos.

Of course, his knack for possessing his killer if they get any satisfaction is easily gotten around. Either by having a Tyranid or Necron kill him, as they can't get satisfaction from the act being emotionless robots or alien locusts. The other solution is if the person who kills him and enjoys it commits suicide before the transformation occurs. OR, take him alive( easier said then done admittedly), cut his limbs off, lock him in a metal box, bury it, build a heavily armed fortress on top full of storm troopers who think there just there to support a crusade and BAM:Problem solved.

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 for obvious reasons). 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. 6th edition hasn't been kind to him, as he's can't do much against TEQs anymore (while Kharn, Abaddon, and even a regular Chaos Lord can do that).