If you'd mention the anime you're using, I could probably be of more help. As just a general reccomendation, if you want real dark, real hard-hitting music, you might want to take a spin around either of Hypocrisy's last two CDs, _Into The Abyss_ and _Catch-22_. A lot of the time extreme metal rhythmics are simply unvideographable (what the hell are you going to do with a blastbeat?), but Hypocrisy sets up a very hardcore death metal sound on a rhythm groove that you can actually do something with.
Hypocrisy records on
Nuclear Blast, which is distributed in the US by
Century Media. You should be able to find them in any decent CD store, and perhaps in Best Buy or other more general outlets.
--K
and oh yeah, check the sig. The Haunted might be a good choice as well.