Saying "recommend me some classical" is like a rock fan saying "recommend me some metal". I haven't a clue where to start with a genre so diverse and with 400 years of history. Most people would say start with the obvious ones, Mozart, Beethoven etc, but I don't care much for either (with a few exceptions). So I'll just list my 5 favourite works, all of which are dark and should appeal to metal fans:
Gorecki: Symphony No 3 (Symphony of Sorrowful songs)
Sibelius: The Swan of Tuonela
Bela Bartok: The Miraculous Mandarin
Michael Nyman: The Piano Concerto
Stravinsky: Firebird