Black eagle Ariel

Crest of Seiros. (3 times/longrest) When activating Ariels crest, she can stun someone with her next attack. The NEXT time she hits someone with a melee attack, the target gains stunned condition until the end of Ariels next turn.

Greataxe. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 6 (1d12 + 4) slashing damage. 

Opportunity Attack. You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack occurs right before the creature leaves your reach