The Beast is an extra-terrestial being and is the main antagonist of Krull. It is a gigantic(in normal form) creature who comes from an unknown planet far from Krull. He travels in The Black Fortress and commands an army of Slayers to conquer the universe.
The Beast travels to Krull as part of his lineage of conquerings of nearby planets. After the Black Fortress lands upon the planet, he immediately sets forth armies of Slayers to conquer the land. After kidnapping Lyssa, and proposing his marriage (at which she refuses), Colwyn finally kills the Beast with his and Lyssa's "love", as well as the Glaive.
The Beast (as humans know him so), is one of many like him, part of an alien race. He travels with his home, Black Fortress, to conquer far off planets with his army of monsters named Slayers, and half-monsters that are Changelings.