SRD:Epic Counterspell

To use epic counterspell, select an opponent as the target. The character does this by readying an action, electing to wait to complete his or her action until the opponent tries to cast a spell (the character may still move his or her speed, because readying a counterspell is a standard action). If the target tries to cast a spell, make a dispel check: Roll d20+40 against a DC of 11 + the foe’s caster level. If the check is successful, the character’s spell negates the foe’s spell.