Who started calling himself Varg Vikernes?  Answer the above question based on the context below:  In the 1980s, a young guitarist called Euronymous forms a black metal band called Mayhem, the first of the genre in their country of Norway, with Necrobutcher on bass, and  Manheim on drums. Manhein leaves and is soon replaced by new drummer Hellhammer and they recruit a new vocalist from Sweden called Dead, who exhibits self-destructive behavior, which he portrays during their live shows by cutting himself and bleeding on the audience, and throwing pig heads at the "posers". At a show filmed by their friend Metalion, the band meets a fan named Kristian, whom Euronymous initially looks down on. While home alone, Dead uses his personal knife to cut his arms and throat, and then uses Euronymous' shotgun to shoot himself in the forehead, leaving behind a suicide note. Euronymous returns home and finds the body but instead of calling the police, he takes photos of the body and moves the knife and shotgun around. After Dead's body is taken away, Euronymous gives necklaces to the other band members which he claims are pieces of Dead's skull; this disgusts Necrobutcher, prompting him to leave the band. Soon after, Euronymous starts his own black metal record label and opens a record shop called Helvete, which becomes a social hub for black-metallers like Metalion, Fenriz of Darkthrone, Faust of Emperor, and Kristian (who is now calling himself Varg Vikernes) of Burzum. They become known as the "Black Circle". After being mocked by an ego-driven Euronymous, Varg uses his anti-Christian beliefs as motivation to burn down a local church. When approached by Varg concerning his status as the leader of the Black Circle, Euronymous burns down a church with Faust and Varg accompanying. Euronymous recruits Varg as bassist, a guitarist called Blackthorn and a Hungarian vocalist, Attila Csihar, to record Mayhem's first album, De Mysteriis Dom Sathanas. A power dispute between Varg and Euronymous arises.
The answer to this question is:
Kristian