Prey on the Weak p. 88
You scythe through weaker foes like a mighty predator turned loose among a herd of prey.
With each foe you strike, your blood- lust and battle fury rises ever higher. After a brief moment of explosive rage, the foes around you are left bloodied, torn, and moaning.
Whenever an opponent within 10 feet of you drops to -1 or fewer hit points, whether from your attack, an ally's strike, or some other cause, you can immediately make an attack of opportunity against any opponent within your threatened area.