Any even-numbered Chopin prelude. Chopin etudes: Op. 10 No. 12 is quite well known and dark. Op. 25 Nos. 11 and 12 are also quite dark, if not quite so well known as the revolutionary. Chopin sonata No. 2 in B-flat minor (every movement is dark, and the third movement is the infamous funeral march). Chopin Ballade No. 4 (my personal favorite).

I could go on and on...

Well, it's not death metal dark and heavy, but Beethoven's Pathetique sonata comes to mind.
Also, when I WAS a metal head, I kind of thought the B minor prelude from WTC book II sort of
rocked (at least at the tempo Glenn Gould played it).