As a reaction, when an enemy makes a ranged attack against
an ally within 60 feet of you, you can warn that ally of the
danger. You must spend your reaction when the enemy
declares the attack but before it makes the attack roll. Your
ally can spend a reaction to gain a +4 to AC against the
triggering attack. Once the triggering attack is resolved, the
ally falls prone.
At 8th level, you can spend 1 Resolve Point to prevent your ally from falling prone after the attack.