Vakoo

Amphibious. The vakoo can breathe air and water.

Innate Spellcasting. The vakoo's spellcasting ability is Charisma (spell save DC 20). It can innately cast the following spells, requiring no material components:

At will: chaos bolt, charm person, dissonant whispers, sleep, summon lesser demons

3/day each: confusion, counterspell, control water, fear  

1/day each: power word pain, summon greater demon

Legendary Resistance (3/Day). If the vakoo fails a saving throw, he can choose to succeed instead.

Magic Resistance. The vakoo has advantage on saving throws against spells and other magical effects.

Magic Weapons. The vakoo's weapon attacks are magical.

Shapechanger. The vakoo can use his action to polymorph into a form that resembles a Medium humanoid, or back into his true form. Aside from its size, its statistics are the same in each form. Any equipment it is wearing or carrying isn’t transformed.

Multiattack. The vakoo makes two shortsword attacks.

Shortsword. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 3) piercing damage, and the target must make a DC 19 Wisdom saving throw. On a failure, the target takes 28 (8d6) psychic damage and becomes frightened until the end of its next turn. On a success the target takes half as much damage and is not frightened.

Beguile (1/Day). The vakoo emits a dazzling display of lights. Each creature within 60 feet of it must succeed on a DC 19 Wisdom saving throw or be charmed for 1 minute. While charmed by this way, the creature is incapacitated and has a speed of 0. The charm ends for an affected creature if it takes any damage or if someone else uses an action to shake the creature out of its stupor.

The vakoo can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The vakoo regains spent legendary actions at the start of its turn.

Cast a Spell. The vakoo casts a spell.

Move. The vakoo moves up to its speed without provoking opportunity attacks.

Invisibility (Costs 2 Actions). The vakoo magically turns invisible until it attacks, casts a spell, uses Beguile. Any equipment the vakoo wears or carries is invisible with it.