Ancient arboreal dragon

Enchantment immunity. The arboreal dragon is immune to all spells in the enchantment school of magic. 

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

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

Alternate form (3/day). The arboreal dragon can transform into any beast, humanoid or fey creature that is size category large or smaller they have seen before and doesn't exceed their own challenge rating. The arboreal dragon's statistics all change to the creature's stats except for their intelligence, wisdom and charisma scores along with their skills. They can stay in this form for a maximum of 10 hours at a time and can't regain hit points in this form but once they are reduced to 0 hit points they revert to their true form. 

Innate Spellcasting. The ancient arboreal dragon's innate spellcasting ability is Charisma (DC 21). The ancient arboreal dragon can innately cast the following spells, requiring no material components:

3/day: Telekinesis 

1/day each: otto's irresistible dance, polymorph 

Multi attack. The ancient arboreal dragon can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +16 to hit, reach 15 ft., 1 target. Hit: 19 (2d10 + 9) piercing damage. 

Claw. Melee Weapon Attack: +16 to hit, reach 5 ft., 1 target. Hit: 15 (2d6 + 9) slashing damage. 

Tail. Melee Weapon Attack: +16 to hit, reach 15 ft., 1 target. Hit: 17 (2d8 + 9) bludgeoning damage. 

Frightful Presence. Each creature of the dragon’s choice that is within 120 feet of the dragon and aware of it must succeed on a DC 21 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature’s saving throw is successful or the effect ends for it, the creature is immune to the dragon’s Frightful Presence for the next 24 hours.

Thorn breath (Recharges 5/6). The arboreal dragon exhales a blast of thorns, each creature within a 90ft cone must make a DC 22 Dexterity saving throw or take 88 (22d8) piercing damage. On a successful saving throw a creature takes half damage. 

The ancient arboreal dragon 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 ancient arboreal dragon regains spent legendary actions at the start of its turn.

Detect. The dragon makes a Wisdom (Perception) check.

Whipping retreat (Costs 3 Actions). The arboreal dragon makes two tail attacks against any creature within range then moves up to their movement speed without taking opportunity attacks and on their next turn they can take the hide action as a bonus action if they end their turn within a dense tree populated area and have a +5 bonus to said hide check.   

Fey spike trap (Costs 3 Actions). The arboreal dragon chooses a space they can see within 60ft that is a 15ft radius that isn't currently occupied. That space's appearance stays the same but for the next 1d4 rounds its an illusion so that when any creature moves into that area they must make 22 Dexterity saving throw or fall into a 5 foot deep pit with spikes at the bottom that dear 32 (8d8) piercing damage. On a successful save the creature avoids the pit and moves to an adjacent square.