Which Finnish heavy metal band released the album 'Once' in 2004?

Nightwish released the 2004 album Once.

Once is the Finnish symphonic-metal band's fifth studio album, released in Finland on 7 June 2004. It was the group's final album to feature original lead vocalist Tarja Turunen and represented a major expansion of Nightwish's orchestral sound.

The band used a full orchestra on nine of the album's eleven tracks, working with the London Philharmonic Orchestra rather than an orchestra based in Finland. The production cost nearly €250,000, making it Finland's most expensive recording at the time; music videos pushed the overall expense even higher.

Once brought Nightwish broader international attention. It topped several European charts, while its lead single “Nemo” became one of the band's most successful songs. The album also includes “Wish I Had an Angel,” “The Siren,” and “Kuolema Tekee Taiteilijan,” a full-length song in Finnish.

The answer is Nightwish, not Children of Bodom, Kalmah, or Ensiferum—other Finnish metal bands that are often grouped with them by country or genre.