Terek is a Deviate and Aura's brother, which Aura didn't even know existed. In the episode Template:116, it is revealed that when Terek was a baby, his father Ming the Merciless, ordered him to be purged (killed), because there was a deviation in his DNA. A woman named Ludi took pity on Terek and smuggled him out of the Citadel. She then raised him as her own son. And by the time he was united with his sister, he had already become the leader of a group of Deviates, living in the remains of a pre-Sorrow city.
After Ming destroyed Terek's home and all the Deviates living there, he vowed to kill Ming.