Vilitch the Curseling
"The child who is not embraced by the village will burn it down to feel its warmth"
- – African proverb
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.
He also showed up during the latest version of Archaon's quest for the Treasures of Chaos, with Archaon helping Vilitch against some Bretonnians in exchange for telling Archaon how to find the Crown of Domination. When the Bretonnians were defeated, Vititch double-crossed Archaon, hoping to loot the treasures Archaon had already gotten. When Archaon turned the tables and held his sword to the Twisted Twin's throats, Vitlich told him the first Daemon Prince Be'lakor would show Archaon the way before Vilitch vanished in a cloud of magical smoke.
Vilitch did little during the End Times, with Archaon sending him to kill Karl Franz and take Averheim, which he failed to do, and he killed Morbidex Twiceborn off screen after Morbidex tried to kill Vilitch for being a follower of Tzeentch (and some trolling from Vilitch here and there).
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.
What happened to Thomin after this is never described - we can assume that either he got blown up with the rest of the Warhammer World, or that he is simply still wandering Tzeentch's realm.
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. It's funny because despite being the only marked Chaos Champion from 8th edition to not die on screen, he's the only that didn't come back for the new setting.
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.
Vilitch and Thomin are back as Tzeentch's representative in the Champions of Chaos DLC where he will be racing the champions of the other gods to the lost city of Zanbaijin to capture the souls within.
Vilitch has a passive ability called Twisted Twin. The more he casts spells or is in melee combat, the better he gets at both casting spells and melee combat.