When a creature makes a melee attack, it can also mark its target. Until the end of the attacker’s next turn, any opportunity attack it makes against the marked target has advantage.
The opportunity attack doesn’t expend the attacker’s reaction, but the attack can’t make the attack if anything, such as the incapacitated condition or the shocking grasp spell, is preventing it from taking reactions.
The attacker is limited to one opportunity attack per turn.
You hurl a bubble of acid. Choose one creature within range, or choose two creatures within range that are within 5 feet of each other. A target must succeed on a Dexterity saving throw or take 1d6 acid damage.
This spells damage increases by 1d6 when you reach 5th Level (2d6), 11th level (3d6) and 17th level (4d6).