Chosen of Tiamat

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

Blessing of the Dragon Queen. If the Chosen of Tiamat takes Acid, Cold, Fire, Lightning, or Poison damage, it gains resistance to damage of that type until the end of its turn. If the Chosen of Tiamat has no resistance from this effect at the start of its turn, it can choose one of the damage types to gain resistance to. The elemental damage dealt by the Chosen's weapon and breath attacks are the same type as this resistance.

Spellcasting. The Chosen of Tiamat is a 17th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 18, +12 to hit with spell attacks). The Chosen of Tiamat has following spells prepared:
At will: Acid Splash, Fire Bolt, Lightning Lure, Poison Spray, Ray of Frost, Shocking Grasp
2/day each: Absorb Elements, Burning Hands, Chaos Bolt, Expeditious Retreat, Misty Step, Fireball, Lightning Bolt.
1/day each:Chain Lightning, Cone of Cold, Fire Storm, Immolation

Wreath of the Prism. When the Chosen hits a beast, dragon, or monstrosity of challenge rating 10 or lower with an attack, or when it grapples it, they can use the wreath to cast dominate monster on the creature (save DC 15). On a successful save, the target is immune to the power of the wreath for 24 hours. On a failure, a shimmering, golden image of the wreath appears as a collar around the target’s neck or as a crown on its head (your choice) until it is no longer charmed by the spell. If you use the wreath to charm a second creature, the first spell immediately ends. When the spell ends, the target knows it was charmed by you.

Multiattack. The Chosen of Tiamat makes three attacks with the Will of the Talon and makes one attack with their Chromatic Breath.

Will of the Talon. Melee Weapon Attack: +15 to hit, reach 5 ft., 1 target. Hit: 14 (1d8 + 8) piercing damage plus 10 (2d10) elemental damage. 

Chromatic Breath. Ranged Breath Weapon Attack: range 30 ft. DC 17 Dexterity Saving Throw. Hit: 27 (5d6 + 10) elemental damage.

Dread Presence: As a bonus action the Chosen of Tiamat emits an echoing roar. Each creature of its choice within 30 feet of it must succeed on a DC 17 Wisdom saving throw or become frightened of it 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 Dread Presence for the next 24 hours.

Chromatic Retreat. When hit by an attack, the Chosen of Tiamat may use a reaction to teleport up to 30 ft away in a burst of elemental power. All enemies within 5 feet of the Chosen must succeed on a DC 17 Constitution saving throw or take 5d6 elemental damage of the same type as its current breath attack.

The Chosen of Tiamat 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 Chosen of Tiamat regains spent legendary actions at the start of its turn.

At-Will Spell (Costs 1 Action): The Chosen of Tiamat casts one of its at-will spells.

Melee Attack (Costs 1 Action): The Chosen of Tiamat makes a melee attack with its Will of the Talon.

Chromatic Fury (Costs 3 Actions): The Chosen of Tiamat unleashes a blast of elemental energy at any number of creatures within 60 feet of it. The targeted creatures must succeed on a DC 20 Constitution saving throw against this effect, taking 10d4 damage of the same type as the current breath attack.