Vitriolic Vengeance

When you are hit by a melee attack by an opponent within 5 feet of you, you may use your reaction to reach out and grasp them, digging your fingers into their flesh, or the joints of their armor, exuding a number of bodily humors mixed into a strong ichor. The opponent then must make a Constitution saving throw. On a failed save, you may expend any amount of HP, and deal your choice of acid or poison damage equal to the amount of expended health divided by 2. On a succeeded save, the opponent forces the mix back into your body, and you take 1d4 acid and 1d4 poison damage.