Needs review
Source: Player's Handbook II

Description: You have learned to channel energy into a foe's spell to weaken its effects.

Prerequisites: Improved Counterspell

You can use an immediate action to attempt to dampen an enemy's spell. As with counterspelling, you must identify the target spell as it is being cast. If you successfully identify it, you can expend any spell or spell slot to dampen your opponent's incantation. Because of your hasty, immediate casting, you do not fully counter the target spell. Instead, you subtract the level of the spell or spell slot you expend from the save DC of your opponent's spell.