This Finnish band formed originally around the gimmick of being a quartet of classically trained cellists doing instrumental covers of Metallica songs.
Early albums were more or less just that -- cello renditions of tunes like "Enter Sandman" and "Creeping Death." By third album Cult, the bandmembers had branched out considerably, adding vocals and drums, and -- most importantly -- writing their own tunes in the aggressive metal vein.
Of course, Apocalyptica couldn't let the original idea go completely, so along with ten driving, demonic tunes of their own, they apply the classical treatment to covers of Metallica's "Fight Fire with Fire" and "Until It Sleeps," as well as offering a rendition of Edvard Grieg's "Hall of the Mountain King.".