Zhentarim Tactics
Prerequisites 4th
You gain the following benefits.
Ability Score Increase. Increase your Dexterity or Charisma by 1, to a maximum of 20.
Retaliate. Immediately after a creature within 5 feet of you hits you with a melee attack, you can make an Opportunity Attack against that creature.
Versatile Merc. When you finish a Long Rest, choose a skill in which you have proficiency. You have Expertise in that skill until you finish your next Long Rest.
Sources
Forgotten Realms: Heroes of Faerûn p. 41