On 21 August 1992, Eithun stabbed Magne Andreassen, a gay man, to death in a forest just outside Lillehammer. Eithun was visiting his family there. He went to a pub and had a drink, but "the atmosphere didn't suit him, so he decided to head home". According to Eithun, while walking in the Olympic park, "this man approached me – he was obviously drunk and obviously a faggot […] it was obvious that he wanted to have some contact. Then he asked me if we could [...] go up to the woods. So I agreed, because already then I had decided that I wanted to kill him, which was very weird because I'm not like this". Eithun carried a knife because, as he explained: "It's better to have a knife you don't need than to not have one when you need it". Once in the woods, Eithun stabbed Andreassen 37 times and then kicked him in the head repeatedly as he lay on the ground.
Eithun claimed that he felt no remorse at the time. In the late 1990s, he said of the murder: "I was outside, just waiting to get out some aggression. It's not easy to describe why it happened. It was meant to happen, and if it was this man or another man, that's not really important". Ihsahn, his bandmate in Emperor, said that Eithun "had been very fascinated by serial killers for a long time, and I guess he wanted to know what it's like to kill a person". The media has linked the murder to black metal and speculated that Eithun was motivated by Satanism or fascism, but in a 2008 interview he explained: "I was never a Satanist or fascist in any way, but I put behind me the hatred and negativity. Those feelings just eat you up from inside". In a 1993 interview he had said "I am not a Satanist, but I praise the evil". In an interview for the book Lords of Chaos he explained he had been "interested in Satanism but there are other things as well. Basically, I don't give a shit". Jørn Tunsberg of the band Hades Almighty said that the murder was "an impulse killing" and that "it had nothing to do with black metal".
Police initially had no suspects, and Eithun remained free for about a year. However, he told Øystein 'Euronymous' Aarseth, Varg Vikernes and a few others what he had done. The day after the stabbing, he returned to Oslo and burnt down Holmenkollen Chapel with Vikernes and Euronymous. After Euronymous's murder in August 1993, Eithun was arrested and confessed to killing Andreassen. In 1994 he was sentenced to 14 years imprisonment, but was released in 2003 after serving nine years and four months.