Light or Dark magic.

Well, maybe the initial dark spells don't require a lot of spell points (although plague does), but the most important dark spells like blindness, berserk, and enthrall really show their power with a good amount of spell points, and unholy word is generally useless.
But you might be right about demons, especially since there's the Fallen Knight. Dark magic would be good against demons, although this needs to be tested in practice, I haven't played that way yet.