Dreaming Water (5e Hazard)
From D&D Wiki
Any area of water can magically become dreaming water, though dreaming water is most common near areas touched by powerful fey magic. It remains still no matter what touches it, and it cannot be fouled by nonmagical means. A creature that comes into direct contact with dreaming water must succeed on a DC 15 Wisdom saving throw against its soporific magic or fall asleep and remain unconscious for 10 minutes. An affected creature awakens if it takes any damage or if another creature uses an action to shake it awake.
Any nonmagical water that comes into contact with dreaming water becomes dreaming water after 1 minute, and dreaming water that is removed from its source and kept at least 30 feet away from it for the same duration becomes nonmagical water.
Dreaming water has no effect on constructs, fey, and undead.