As an enemy reels from a terrible wound, you quickly loose an arrow to finish him off.

Encounter   ✦     Martial, Weapon
Immediate Reaction      Ranged weapon

Trigger: One creature you can see has just taken damage

Target: The creature that took damage

Attack: Dexterity vs. AC

Hit: 2[W] + Dexterity modifier damage, or 3[W] + Dexterity modifier damage if the target is bloodied.

Published in Player's Handbook, page(s) 113.