Blood calls to blood. Your pain and your enemy's give strength to your assault.

Encounter   ✦     Primal, Weapon
Standard Action      Melee weapon

Target: One creature

Attack: Strength vs. AC

Hit: 2[W] + Strength modifier damage. If you or the target is bloodied, the attack deals 1[W] extra damage.

Published in Player's Handbook 2, page(s) 52.