Empress Metallia is the primary villian of the first season of the Sailor Moon series. Metallia herself is an undefined entity of some form of evil energy. In her true form, she appears as an amorphous cloud of shadow and evil red light, with a vaguely human face. Her powers are apocalyptic in scale, with the ability to destroy the entire Moon Kingdom civilization in a single attack. Metallia was sealed away in her moment of triumph by the Imperium Silver Crystal, and has since been trapped in a cave deep under the arctic icecap.
Metallia's true origins and motivations besides general malevolence are never explained in the original series, nor is the extent of her full power displayed. She is apparently the source of all the youma, can possess other beings and needs a great deal of energy to be released. Metallia is probably one of the 'Children of Chaos' that are referred to during the final season of Sailor Moon, along with such beings as the Death Phantom and Pharoah 90.