Vengeful guardian is a class feature given to the Berserker class.
Vengeful Guardian
Barbarian attack
Trigger: "An enemy subject to your defender aura either shifts or makes an attack that targets you or an ally who has an active defender aura." [HotF:44]
Effect: "You make a melee basic attack against the triggering enemy and deal 1d8 extra damage to it.
level 11: 2d8 extra damage.