| This spell calls forth a mighty storm of roaring flame directly onto the target, dealing partially irresistible fire damage in a large area and leaving behind short-lived fire vortices.
“Some have said there is no subtlety to destruction. You know what? They're dead.”
-Jaya Ballard, task mage (Magic: the Gathering)
Fire Storm is a 9th-level Fire Magic/Conjurations spell that can be targeted on a visible, open square (occupied or not) in a 6-square range. It creates a fiery conflagration of random size (3-4 tiles in radius including the center tile) and fills it with clouds of fire, similar to those created by Conjure Flame, and summons a handful of fire vortices.
Fire Storm's damage potential is one of the highest in the game (8d((5+Spell power)/8), up to 208 damage per turn at max spellpower), and only 55% resistible: you can blast all kinds of fire-resistant monsters with it (even the dreaded orb of fire!) and they'll still take damage, though less. Since it uses smite-targeting, you kill monsters that are invisible, around corners, outside of line of sight, and even on the other side of translucent walls.
The vortices Fire Storm creates are weak and confused friendly monsters that wander around, meleeing enemies for a bit of additional damage. They have low HP and die after scoring a single hit, but can serve as a distraction and even get in the way of projectiles or spells.
On the flip side, Fire Storm is incredibly difficult to learn. It's only found in the Book of Annihilations, and you'll need Fire Magic and Conjurations skills of at least 18, with a staff or ring of wizardry equipped, to be able to learn it and cast it reliably, and its hunger and MP costs are tremendous. Also, casting Fire Storm causes a real ruckus, akin to shouting; you may get swarmed when you really don't want to.
Fire Storm's random radius makes it difficult to target monsters that are close to you without hitting yourself in the process - a bad idea, given that even with rF+++, you'll still be taking about 8d12 or 8d13 damage. Blink, or Controlled Blink if you can, complements Fire Storm nicely. Keep some single-target damage spells in your repertoire for monsters that don't deserve 9 MP worth of fiery death.