Necro invented the term "death rap" to describe his style of ultraviolent hip-hop and to set himself apart from other genre labels created by the media. This sub-genre of rap combines death metal beats with explicit raps about violence, death, the occult, and sex.
-Wikipedia

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If heavy metal and rap were to come together, this is where they'd probably meet.
I only discovered Necro recently, but I'd say my favorites are Chasing the Dragon ft. Ill Bill (hip-hop ver.) and Beautiful Music to Die To.

Chasing the Dragon
http://www.youtube.com/watch?feature=player_detailpage&v=PpjnRm7gPqQ

Beautiful Music to Die To
http://www.youtube.com/watch?v=PA2yfUFMnGw