Thunderfury Rage is a daily power available to Barbarians at 15th level.[PH2:57]
Thunderfury Rage
Barbarian Attack 15
Charging at your foe and knocking it to the ground, you call on the spirit of the thunderfury boar to drive you into a frenzy. In your rage, you lash out at foes that attack you.
Melee weapon
Target: one creature
Attack: Strength vs AC
Hit: 3[W] + Strength modifier damage and you knock that target prone.
Miss: Half damage.
Effect: You enter the rage of the thunderfury boar. Until the rage ends, when any enemy adjacent to you hits or misses you, you can make a melee basic attack against that enemy as an immediate reaction.