From Lexicanum

Once a Captain of the Thousand Sons, Karlsen became a Champion of Tzeentch and a veteran of the Long War, having fought against the Imperium since the Horus Heresy. A Sorcerer Karlsen took part in nearly all of his Legion's significant history, including the fall of Prospero, the Battle of Terra and countless other combat actions. His lifespan unnaturally extended by his Chaotic patron, Karlsen went completely insane on more than one occasion due to the burden of his experiences, before he invented a ritual to 'partition' his mind and safely store his memories. Karlsen has noticeable mutations: one of his hands is fused to his bolter and he operates the weapon by mental control, whilst the fingers of his other hand have lengthened to become almost tentacle-like.¹

Sources

• ¹:Warhammer 40,000: Wargear (2nd Edition), p.78-80