Potion, legendary
This vial is filled with an opalescent liquid, and demonic faces can occasionally be seen swirling within. The potion is odorless and tasteless. After you consume it, the next time you fall asleep, you are trapped within nightmares from which you can’t be woken for 8 hours. Upon waking, you must make a DC 17 Wisdom saving throw. On a failed save, you take 3d6 psychic damage and don’t gain any benefit from your long rest. On a successful save, you are shaken by the nightmares and have disadvantage on ability checks made within 8 hours of waking up. This potion has no effect on creatures that do not sleep or dream.