You are in the key of E minor; a very common metal key. Most solos are in the aeolian or pentatonic scales, but because you play an F5 power chord, that means you should use E Phrygian, an exotic, mmiddle-eastern, yet evil mode.

Go here to see the shape for phrygian:
http://pages.prodigy.net/aj5000

If you really want evil, use Locrian, and throw a Bb5 (B flat) power chord into the progression. From E to Bb is a diminished 5th (or a tritone), the most evil interval of them all. They used to hang minstrels in medieval times for playing it in front of the king...