Drone

The creature produces a horrid droning sound to which demons are immune. Any other target that starts its turn within 30 feet of the creature must succeed on a Constitution saving throw (DC = 8 + proficiency bonus + Constitution modifier) or fall unconscious for 10 minutes. A target that can't hear the drone automatically succeeds on the save. The effect on the target ends if it takes damage or if another target takes an action to splash it with holy water. If a target's saving throw is successful or the effect ends for it, it is immune to the drone for the next 24 hours. (Chasme, pp. 57, MM)