You force your enemy to feel the pain that it inflicts on your friend.

At-Will   ✦     Force, Psionic, Psychic
Immediate Reaction      Melee 1

Trigger: An adjacent enemy marked by you deals damage to your ally with an attack that doesn't include you as a target

Target: The triggering enemy

Effect: The target takes force and psychic damage equal to the damage that its attack dealt to your ally.

Published in Player's Handbook 3, page(s) 44.