An off-hand attack your foe never saw coming causes disorientation as well as pain.

Encounter   ✦     Martial, Weapon
Immediate Reaction      Melee weapon

Requirement: You must be wielding two melee weapons.

Trigger: You make an opportunity attack against an enemy.

Target: The triggering enemy

Attack: Strength vs. AC (off-hand weapon)

Hit: 2[W] (off-hand) + Strength modifier damage. Until the end of your next turn, you also gain combat advantage and concealment against the target.

Published in Dragon Magazine 384, page(s) 36.