Gregorian chant is a form of plainchant sung in Latin in medieval Roman Catholic liturgy, named after Pope Gregory I, and is known for single line melody without harmony or instruments. In gaming, this style became widely recognised through the main theme for Halo: Combat Evolved from 2001, composed by Martin O'Donnell and Michael Salvatori. Its chant opening returned in Halo: Combat Evolved Anniversary, and short clips spread online as a shorthand for something divine, powerful or absurdly important appearing on screen.