Cloud Form. The cloud can move through a space as narrow as 1 inch wide without squeezing.

Evasion. If the cloud is subjected to an effect that allows it to make a Dexterity saving throw to take only half damage, the cloud instead takes no damage if it succeeds on the saving throw, and only half damage if it fails.

Innate Spellcasting (1/Day). The cloud can innately cast feeblemind (spell save DC 18), requiring no material components. Its innate spellcasting ability is Intelligence. 

Legendary Resistance (3/day). If the cloud fails a saving throw, it can choose to succeed instead.

Mindless Babble. The cloud gibbers incoherently if it isn't incapacitated. Each creature that starts its turn within 30 feet of the cloud and can hear the gibbering must succeed on a DC 18 Wisdom saving throw. On a failure, the creature can't take reactions until the start of its next turn and rolls a d8 to determine what it does during its turn. On a 1 to 4, the creature does nothing. On a 5 or 6, the creature takes no action or bonus action and uses all its movement to move in a randomly determined direction. On a 7 or 8, the creature makes a melee attack against a randomly determined creature within its reach or does nothing if it can't make such an attack.

Multiattack. The gibbering mouther makes two mind strike attacks and one dusk ray attack.

Mind Strike. Melee Spell Attack: +10 to hit, reach 5 ft., one target. Hit: 17 (2d10 + 6) psychic damage, and if the target is a creature, it must make a DC 18 Wisdom saving throw or be incapacitated until the end of the cloud's next turn.

Dusk Ray. Ranged Spell Attack: +10 to hit, reach 60 ft., one target. Hit:  16 (3d6 + 6) necrotic damage.