Ancestral War Band Rage is a daily power available to Barbarians at level 15.[PrP:17]
Ancestral War Band Rage
Barbarian Attack 15
You hew at your enemy, and the spirits of ancestral warrior rise up to aid you.
Melee weapon
Target: one creature
Attack: Strength vs AC
Hit: 4[W] + Strength modifier damage
Miss: Half damage.
Effect: You enter the rage of the ancestral war band. Until the rage ends, you gain combat advantage against any enemy you make a primal melee attack against. In addition, whenever you roll a 1 on a damage die or a primal attack, the roll changes to the die's maximum value.