Your warning shout allows your ally to turn an opening into an advantage.

Encounter   ✦     Martial
Immediate Interrupt      Close burst 5

Trigger: An enemy within 5 squares of you hits an ally with an opportunity attack

Target: The triggering ally in the burst

Effect: The target makes a melee basic attack against the attacking enemy as a free action. If this attack hits, the attacking enemy rerolls the opportunity attack against a different creature of your choice within its reach.

  Resourceful Presence: The enemy gains a bonus to the new attack and damage rolls equal to your Charisma modifier.

Published in Martial Power, page(s) 110.