Be very carefull when using Firestorm.
The spell is more than awesome (and it should considering how difficult it is to be able to learn it then cast it reliably).
But i don't know if it's a bug or a design decision, but the reflection brand of an item is able to reflect the damage back to you (despite it is a smite-targeted spell).
And Firestorm does a lot of damage, a very lot, even with Fire Resistance level 2.
I was toying with a ziggurat in which Firestorm has proved its worth against packs of monsters sticking together (and Vehumet giving back tons of magic as lots of monsters were dying each turns, out of the food problem i could cast it non stop as the killed monsters were enough to bring enough MP to get back to my max MP), until a level that was overpopulated by Deep Dwarves.
Considering there was a big bunch of them packed together, i decided to cast logically Firestorm.
[...]
You feel a bit more experienced.
The great blast of fire engulfs the deep dwarf death knight.
The deep dwarf death knight is almost dead.
Vehumet accepts your kill.
You feel a bit more experienced. It reflects your damage back at you!
Ouch! That really hurt!
You die...
and
Killed by a reflected bolt (56 damage)
... in level 13 of a ziggurat on May 7, 2011.
And that despite having 2 level of Fire Resistance.
So be very carefull if it's not a bug that smite-like spells are affected by the Reflection brand.