Fire Blast is the spell the Achaean God uses against you, and is also the spell cast by the Wand of Disintegrate. You must return your first Wand of Disintegrate as part of the Lost City quest, but it will become available in the store at the end of the game.
The Fire Blast projectile has inherent homing abilities and a very large explosion radius. The projectile is very fast, so using it in confined spaces is highly dangerous, and may deal as much damage to the caster as any enemies present.
Each hit deals roughly 20 fire damage.
At 15 mana per cast, the damage efficiency of this spell is 1.3 dpm.
Fire Blast is available to all classes via the Wand of Disintegrate, though it's unlikely any but the Priest and the Mage will have the mana to effectively use it.