Unparalleled Duelist (5e Feat)
From D&D Wiki
Prerequisites: 13 Dexterity
When you are within 5 feet of 1 creature and no other creatures, you may use a bonus action to mark them as your Duelist's Quarry. While you remain within 5 feet of them at the end of each turn, and attack no other enemies, your first melee attack you make against your quarry each turn has advantage.
When your quarry attempts to use a disengage action, you may attempt to make an opportunity attack against them as normal.