When you counter a spell, the you can confuse the thwarted caster.
Benefit: Whenever you successfully counter a spell, you may cause the caster of the countered spell to become confused for a number of rounds equal to your Charisma modifier (minimum 1). Your caster level for this effect is equal to your caster level. You may use this effect a number of times per day equal to your arcane trickster level.
Undefeatable: The Collected Feats Sourcebook, Copyright 2009 – 2010, Louis Porter Jr. Design, Inc. Undefeated, Copyright 2011, Louis Porter Jr. Design, Inc.