Lucius was a ten-thousand-year veteran, a traitor who had taken part in the Horus Heresy.
Before the rebellion, Lucius had served in the Emperor's Children Chapter under the Gene-Primarch Fulgrim.
When the God-Emperor created the Gene-Primarchs, he had once made an agreement with the Four Chaos Gods, obtaining some kind of Warp Essence from them to create Lesser Gods.
Among them, Gene-Primarch Fulgrim had been chosen in advance by Slaanesh.
Against that backdrop, a large portion of the Emperor's Children Chapter had been marked by Slaanesh, Lucius included.
While still on the loyalist side, Lucius had been an extremely powerful swordsman and had served as captain of the 13th Company during the Great Crusade.
But he had a flaw: extreme arrogance and jealousy.
Like a bully in American-style bullying, he often ran his mouth at members of other Chapters, calling them country bumpkins.
He was absurdly conceited and supremely confident in his swordsmanship. If anyone was better with a blade than him, more handsome than him, or received more praise than him, he would find a way to kill them.
In short, he was both arrogant and jealous, and his swordsmanship truly was exceptional—exactly Slaanesh's type.
So, after Lucius was killed once, Slaanesh bestowed its blessing upon him, resurrecting him and granting him the power of immortality.
So long as whoever killed Lucius in battle felt even the slightest pleasure or sense of accomplishment, Lucius would gradually resurrect from within that person's body and slowly take possession of them.
Lucius's possession-based resurrection ability was utterly absurd. For example:
Lucius once stepped on a landmine and was blown to death. A thousand miles away, a factory worker felt proud for assembling landmines for the Imperium—and was then possessed by him.
Lucius was struck by a Necron disintegration ray and turned into