Therion is a Swedish symphonic metal band founded by Christofer Johnsson in 1987. Its name was inspired by the Celtic Frost album To Mega Therion. "To Mega Therion" is Greek for "The Great Beast" and was a title used by occultist Aleister Crowley. Originally a death metal band, Therion adjusted its musical style by adding orchestral elements, including choirs, classical musicians, and even a full orchestra at their concert performances. As a result, they are considered pioneers of the symphonic metal genre.

Symphonic metal is a cross-generic style designation for the symphonic subsets of heavy metal music subgenres. It is used to denote any metal band that makes use of symphonic or orchestral elements. The style features the heavy drums and guitars of metal with different elements of orchestral classical music, such as symphonic instruments, choirs and sometimes a full orchestra, or just keyboard orchestration.