You combine fire with one of your allies to take down a troublesome foe.

Encounter   ✦     Martial, Weapon
Immediate Reaction      Ranged weapon

Trigger: An ally makes a ranged attack or an area attack

Target: One creature attacked by your ally

Attack: Dexterity vs. AC

Hit: 3[W] + Dexterity modifier damage.

Published in Player's Handbook, page(s) 114.