Thallik Wormfather was a necromancer in the Worm Cult. During the Three Banners War, he went to The Rift with a group of cultists to raise Sinmur, the giant king slain by Ysgramor and the Five Hundred Companions. First, he convinced an army of Reachmen to attack Shor's Stone to distract the Companions and the Ebonheart Pact army from his true objective: stealing the axe Wuuthrad from the catacombs beneath Fallowstone Hall. His diversion succeeded, and he escaped with Wuuthrad just as Pact forces led by the Soulless One attacked the Hall.
Thallik then raised Sinmur from the dead by using the power contained within Wuuthrad, shattering it into several pieces. He was killed by the Soulless One, but merged his spirit within Sinmur, making the undead giant nearly invulnerable to harm. Sinmur then rampaged through Riften, destroying the northern part of the city before making his way to the Daedric ruins of Trollhetta in the mountains with the remaining Worm Cult forces.
High King Jorunn and the Vestige led an attack with Pact troops and the Companions, routing the remaining Worm Cult forces and fought the combined Sinmur/Thallik, weakening him using the shards of Wuuthrad. They were able to separate the two spirits and slay Thallik and Sinmur.