Mustis made his first appearance on a Dimmu Borgir release when he played keyboards on live tracks featured in the compilation album Godless Savage Garden (1998).

His other studio work with Dimmu Borgir includes Spiritual Black Dimensions (1999), Puritanical Euphoric Misanthropia (2001), Death Cult Armageddon (2003), the re-release of Stormblåst (2005) and In Sorte Diaboli (2007).

Likewise, according to his statement, he was fired by text message and he will be allowing "the resolution come from the lawyers instead of dealing with more dishonesty and lies on a one-on-one personal level".

The lead single, "Nothing Remains", was entered into 2011's Eurovision Song Contest to represent Norway, but it was knocked out in the semi-final of the qualifying Melodi Grand Prix.

[6] After 15 years, he reunited with Dimmu Borgir to perform "The Insight and the Catharsis" at the Inferno Metal Festival 2024, along with other former bandmates ICS Vortex and Tjodalv.