The first time an enemy attacks you, that opponent discovers just how bad an idea that is.

Daily   ✦     Martial
Immediate Interrupt      Personal

Trigger: An adjacent enemy makes a melee attack against you for the first time during this encounter, and you are not granting combat advantage.

Effect: You gain a +10 power bonus to all defenses against the enemy's attack. If the attack misses, the enemy takes double your Shadow Assassin's Riposte damage.

Update (10/4/2011)
Updated in Class Compendium.

Published in Player's Handbook, page(s) 128, Class Compendium.