Vilitch the Curseling
Vilitch and Thomin were twins, whose births killed their mother. Where Thomin was healthy, strong, and good-looking, Vilitch was the opposite, being frail, weak, and ugly. Growing up, Vilitch was forced to do the chores of his dead mother and suffered beatings from Thomin. Thomin was their father's favorite, so he allowed Vilitch to be beaten, and the latter was also forbidden from using a sword. As they grew up, Thomin became a great warrior, while Vilitch was barely able to get by as the local shaman's apprentice and every night he prayed to Tzeentch to reverse the fates of him and his twin. And one night, as Morrslieb was close to the world, the god of change agreed to grant his request.
The next morning, Vilitch woke up to discover that he had melded with his twin, and that Thomin was now naught but an automaton under his command while he had all sorts of forbidden knowledge. With his enhanced intellect and sorcerous power, he strode out and destroyed his village and killed its inhabitants. From there he hunted down the best warriors of his village and enslaved their minds, making them his puppets.
His fate following the world-ending was little better than some of his colleagues who got wiped from existence. In attempt to flee after failing to kill Karl Franz, Vilitch managed to enter the Realms of Chaos. While wandering through the Tzeentchian part, he encountered a being that may or may not have been Tzeentch that told him his prayer would be answered. When Vilitch denied making any prayer, he heard a voice saying that it had - Thomin's. Somehow, his brother Thomin had not only regained his intelligence but had made a prayer to be saved from being Vilitch's tool. Thomin gained control of their combined body while Vilitch's intelligence ebbed away until he was reduced to a tagalong stitched to his brother, his only purpose casting the magic Thomin now controlled.
Age of Sigmar
Vilitch lives on (or at least his model does) through a Tzeentch Arcanite unit named, appropriately, the Curseling. It's basically a normal Tzeentch cultist that learns too many secrets in his time, and grows a creepy chaos parasite called a Tretchlet on his back. The Tretchlet whispers secrets into the Curseling's ear, meaning that they make great interrogators.
Gameplay wise, he's an alright wizard assuming the enemy has no magic, but a great pick against magic heavy lists. His ability to "eat the secrets" of enemy mages means every time he unbounds a spell, he can automatically cast it.