Darksoul

When the Shadowlord descended on Mordheim, his presence attracted the most despicable men and women in the Empire, people willing to sacrifice their very souls for the promise of real power. By pledging their service to the Ruinous Powers, these individuals had their souls blasted away and becoming something else, something far darker and sinister. They become Darksouls. Once so possessed, they gained incredible power fuelled by the Daemonic energies coursing within them, but they were sanity-blasted by the experience of having their souls devoured by the hostile presence within. Strangely, the Daemons did not remain and left these individuals as empty, insane husks who want nothing more than to kill.

The terrible method of creating Darksouls has survived over the centuries, continued by the savage peoples of Norsca and the Eastern Steppe. Through a perverse ritual, they bind the mortal inside a summoning circle where the victim serves as the conduit for a conjured Daemon. Once the ritual is complete, instead of producing the Daemon bodily, it manifests within the mortal.

Darksouls appear as ordinary Humans, but looking closely at their dilated eyes and listening to the endless blasphemies spilling from their mouths dispels any doubts about their nature. Older Darksouls are barely recognisable; their bodies are scarred and disfigured by the terrible energies of the Daemons that once possessed them. Interestingly, the Daemons never remain long enough to mutate the host—only to drive them mad. Most Darksouls look like wild and untamed warriors, smeared with blood, dirt, and excrement, wielding jagged weapons and dressing in vile skins (sometimes Human) and rusted armour. Some Darksouls wear Daemonic masks and armour to remind them of their beloved masters.

Becoming a Darksoul
Darksouls no longer house the essence of the Daemon, but are deeply scarred by the experience. The Hung and Kurgan regularly subject captured slaves to terrifying rituals in which they bind a Daemon into their captive, letting it work its evil on the mortal form just long enough to destroy whatever good remained. Once the Daemon has done its work, they banish the creature and welcome the newly born Darksoul into their midst. Those who become Darksouls are forever changed, their Humanity stripped from them for all time. They become wild and crazed creatures, retaining their Human form but being of an utterly inhuman mind.

Source

 * Warhammer Fantasy RPG 2nd ED -- Tome of Corruption (pg. 156-157)