Consume Magic was one of the priest racial abilities available only to blood elfpriests. It allowed the priest to consume a priest-specific buff on themselves in order to regain mana. This was very useful as part of a priest's mana regen rotation.
This ability allowed a priest to consume one of his or her own buffs, such as [Inner Fire], [Touch of Weakness], [Power Word: Fortitude], and so on in order to regain mana. In long mana intensive fights, this could be incredibly useful. The mana returned scaled with the priest's level as well.
Tips and tactics
Place a Rank 1 Inner Fire (or Touch of Weakness) on yourself, and then use Consume Magic. If the proper buff gets consumed, then you have just regained a good chunk of mana at almost no cost. However, it is advisable to not interrupt mana regen; instead, Consume Magic should be used in between casts.