Karlsson is also the lyricist of symphonic metal band Therion, and has written almost all the band's lyrics since 1996. He also provided the whispers on the Therion song Ljusalfheim on their Secret of the Runes album. Nevertheless, Karlsson was never regarded as an official band member.
Karlsson is the lead vocalist of the symphonic death metal band Shadowseeds along with fellow Dragon Rouge member Tommie Eriksson, who also was Therion's guitarist and drummer. The lyrics reflect the Order's teachings and rituals. The band is currently on hiatus.