Ancient Oni

Innate Spellcasting. The ancient oni's innate spellcasting ability is Charisma (spell save DC 19). The ancient oni can innately cast the following spells, requiring no material components:

At will: darkness, greater invisibilityinvisibility

2/day each: dominate monster, cone of cold, gaseous form, sleep, misty step

Magic Weapons. The ancient oni's weapon attacks are magical.

Regeneration. The ancient oni regains 20 hit points at the start of its turn if it has at least 1 hit point.

Aura of Fear. The ancient oni terrifies enemies with its mere prescence. Any hostile  creature that is not currently frightened and starts its turn within 20 feet of the ancient oni must make a DC 19 wisdom saving throw or be frightened for 1 minute. When frightened, the creature must choose to either fight or flight. If they flight, they must do all in their power to get as far away from the ancient oni as possible. They cannot stop until they get to a place they think of as "safe". Once they get to a place they think is "safe", they can repeat the saving throw. If they fight, they no longer are frightened and must use their action each turn to attack the ancient oni or harm it in some way. If they do more than 20 damage to it, they can repeat the saving throw. They are still subject to the fear aura and if they fail again, they are frightened and must choose to flight. If they succeed, they are immune to the Aura of Fear of all ancient onis for the next 24 hours. 

Multiattack. The ancient oni makes two attacks, which can be any mix of claws, bites, and glaives, and then uses Nursery Rhyme once if availible. 

Claw (Oni Form Only). Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 20 (2d12 +7) slashing damage.

Bite (Oni Form Only). Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 13 (1d12 +7) piercing damage and 11 (2d10); necrotic damage.

+1 Glaive. Melee Weapon Attack: +13 to hit, reach 15 ft., one target. Hit: 19 (2d10 + 8) slashing damage, or 13 (1d10 + 8) slashing damage in Small or Medium form.

Nursery Rhyme (Recharge 6). The ancient oni sings an ancient nursery rhyme about the oni. All creatures that can hear it must make a DC 19 wisdom or charisma saving throw (their choice) or take 5d10 psycic damage on a failure, and half on a success. If they fail, they are also under the following effects for 10 minutes: Their vision is limited to 20 feet, no matter what senses they have, as mystical illusionary darkness appears all around them. Only they can percieve it, the darkness isn't actually there. It is a blockage in their mind, limiting their perception of the world around them. If it is nighttime, the power of the darkness is amplified and they are effectively blinded from it. They can repeat the saving throw if they take damage, hear any music or song, or if a spell is cast on them that can reasonably help, like lesser restoration, dispel magic, or something similar. On a success, they are immune to ancient oni nursery rhymes for the next 24 hours.

Change Shape. The ancient oni magically polymorphs into a Small or Medium humanoid, into a Large giant, or back into its true form. Other than its size, its statistics are the same in each form. The only equipment that is transformed is its glaive, which shrinks so that it can be wielded in humanoid form. If the oni dies, it reverts to its true form, and its glaive reverts to its normal size.

Ancient oni can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. The ancient oni regains spent legendary actions at the start of their turn.

Cast At-Will Spell. The ancient oni casts a spell it can cast at will. 

Detect. The ancient oni makes a Wisdom (Perception) check.

Misty Escape. The ancient oni casts misty step or gaseus form, using its innate spellcasting uses of those spells.

Slice n' Dice. The ancient oni makes a +1 glaive attack or a claw attack.