Im planning on writing a Death Metal/Grindcore song and I was wondering which mode would be best to use... Im thinking either Phrygian or Locrain because they are the darkest sounding... But I was just hoping some of you could give me suggestions.

Those two are good. Try experimentin with phrygian dominant too (like phrygian with a major third), and some chromatics too.
Thanks :]
Uhh soo Phrygian is of course HWWWHWW...
Im not familiar with the "dominants".
Ill be playing in D Phrygian which will be D D#/Eb F G A A#/Bb C..

So what would be the Major third (To make it Dominant) ?