Amount of damage dealt by Exorcism is increased by 42% of your Spell power.
Exorcism is one of the few ranged attacks available to Paladins.
Patch 3.1.0 (2009-04-14): Exorcism: Now can be used on any target and has a 100% chance to be a critical strike when used on Undead and Demons.
This is to ensure that paladins do not see a drop in their dps when progressing from Naxxramas (which is full of undead) to Ulduar and beyond (not as much undead). Due to this change, [Glyph of Exorcism] was changed to 20% bonus damage.