The Mask (also known in the comics as Big Head) is the codename for people who are endowed with superpowers and become dangerously violent after donning the Mask of Loki. The wearer of the Mask sports a large, green, bald head and large teeth. The Mask grants powers that allow the wearer to do nearly anything he or she wants. The Mask/Big Head is essentially a living killer cartoon.
In the movies and the animated series, the Mask shows far more restraint than in the comic books. The only person the Mask apparently kills in the first film is Dorian Tyrell. Even then, Dorian is flushed away and does not actually die onscreen.