For the undead, there is often no other way their undeath begins other than to have life channeled through their corpse. Illtas was a different case. He is not quite undead, but not quite alive. He is what is known as a Dread Lord, which only 2 have ever existed. While they live, they possess both powers of liches and death knights. When they die, they turn into wraiths, undead who live within crypts. Illtas, however, was never a wraith.
The other Dread Lord called himself Klargisz. He was closest to both Arrek and Imperfect. Illtas, who was almost never any of their allies, prefered to ally himself with the Reapers. Both Illtas and Klargisz were humans who succumbed to the Black Tears.
Illtas contracted this disease when he fought a necromancer. The pre-Dread Lord was seeking his path as a warrior when he encountered that necromancer. They engaged within battle, and by the end of it, Illtas barely clinged onto his life. That was when the tears began to drip. The black liquid escaped his eyes, and when it hit the gravel road beneath him, something changed. The whole mood of the green forest seemed to drop a shade. The hue slightly faded. Just at that moment, the necromancer had realized what he had done. By that time, it was too late.
As Illtas walked away from the event, he saw that his fingers felt surprisingly light. He looked down to noticed they had turned skeletal. He then noticed his skin and flesh was fading into non existence. His screams were heard across the Mali Mountains and beyond.
Now, he sits upon his throne, all signs of skin faded away. All that is left is his armor, now turned a darkish blue color, his blade, an entirely pale sword meant for two hands, but is only held in one, and his bones. He commands the undead armies of southern Arrekia, and he is the evil that turns the world into a horrific landscape slowly, centimeter by centimeter.
However, when Imperfect slew him, he did not turn into a wraith. He instead was absorbed by Imperfect, throwing all evil into a chaotic balance instead of the controlled one.