Ah, Death Metal (allmusic, wikipedia)...now here's a subgenre that's pretty much guaranteed to freak out your parents and touch off a moral panic among your Sunday School teachers.  This stuff is heavy, harsh, with virtually indecipherable vocals.  Indeed, it's the guttural, growled vocals that distinguish Death Metal in my mind from other subgenres.  In a lot of ways, it seems to me that the performances of Death Metal vocalists are more of a supplemental instrumentation than anything else.  Death Metal also tends to vary the tempo quite a bit within tracks...this can be rather unsettling to those new to Metal, and I imagine that's part of the point; Death Metal is intentionally shocking and disturbing.  Your grandparents will probably not enjoy this music very much.