Your fierce attack brings you into your opponent's reach, ensuring that it can't attack you or escape without reprisal.

Encounter   ✦     Primal, Weapon
Standard Action      Melee weapon

Target: One creature

Attack: Strength vs. AC

Hit: 3[W] + Strength modifier damage.

Effect: Whenever the target shifts or makes an attack roll against you before the end of your next turn, you can make an opportunity attack against it.

Published in Player's Handbook 2, page(s) 58.