One of the first serious metal bands to hire an actual opera singer and orchestra was the progressive metal band Celtic Frost, on the album Into the Pandemonium. Many others experimented with the idea, but bands using full-time classical vocalists did not surface until the late 1990s. Modern examples are Nightwish, Tristania, Therion, Third and the Mortal, and Theatre of Tragedy.
Bands of this genre typically combine a female opera singer with a male vocalist using a more traditional heavy metal style. Some, such as Therion and Nightwish, hire full orchestras while others, such as Tristania, substitute keyboards for economic reasons. Most use a few classical solo instruments to add melodic complexity to their work.