Black Dragon Oil

The viscous green-black oil within this magical ceramic pot bubbles slightly. The pot's stone stopper is sealed with greasy, dark wax. The pot contains 5 ounces of pure black dragon essence, obtained by slowly boiling the dragon in its own acidic secretions. You can use an action to apply 1 ounce of the oil to a weapon or single piece of ammunition. The next attack made with that weapon or ammunition deals an extra 2d8 acid damage to the target. A creature that takes the acid damage must succeed on a DC 15 Constitution saving throw at the start of its next turn or be burned for an extra 2d8 acid damage.