Awesome spell: Magma Eruption.
Everybody enthuses about Blade Wind, but somehow Blade Wind doesn't satisfy my craving for wanton destruction. On the other hand, I love seeing Magma Eruption blow gaping holes in charging barbarian hordes. AOE 5 (+1 per extra level) and dmg 23 (+1), not resistable with fire resistance; it is so very, very satisfying, even with relatively poor precision on the units that tend to be able to cast it. Would you rather moderately wound 10 units with Blade Wind, or kill 6 with Magma Eruption? And the fatigue cost is lower, too.
-Max
P.S. See the battle in Uglanda, province #122, in the saved game at
http://students.cs.byu.edu/~mdw45/late_agartha.trn for an example of what makes me cackle.