Player's Handbook
You have mastered techniques to take advantage of every drop in any enemy's guard, gaining the following benefits.
- When you hit a creature with an Opportunity Attack, the creature's speed becomes 0 for the rest of the turn.
- Creatures provoke Opportunity Attack from you even if they take the Disengage Action before leaving your reach.
- When a creature within 5 feet of you makes an Attack against a target other than you (and that target doesn't have this feat), you can use your Reaction to make a melee weapon attack against the attacking creature.