In Death Masks, his appearance is similar to a grizzly bear with six legs, curling rams horns, and four eyes. One pair of the eyes glowed with faint orange light, and the other with green light. He had two rows of serrated, slime-coated teeth. He had a luminous swirling tattoo on his forehead.
Harry Dresden accidentally soulgazed Rasmussen, and determined that he was a prisoner in his own soul. He was one of the poor people who had their free will gradually stripped away from them by their Fallen.