Iron Breaker's Shout is an encounter power available to Barbarians at 13th level.[PrP:17]
Iron Breaker's Shout
Barbarian Attack 13
You deliver a telling blow, then issue a battle cry so ferocious that is causes your foes' defenses to weaken.
Melee weapon
Target: one creature
Attack: Strength vs AC
Hit: 2[W] + Strength modifier damage. You then howl in a close blast 3 that includes the target. Each enemy in blast takes a penalty to all defenses equal to your Constitution modifier until the end of your next turn
Thunderborn Wrath: The penalty to all defenses equals 1+ your Constitution modifier.