Tiamat, Queen of Dragons

Chromatic Wrath (Recharges after a Short or Long Rest). If Tiamat would be reduced to 0 hit points, her current hit point total instead resets to 600 hit points, she immediately uses a breath weapon of her choice, and she regains any expended uses of Legendary Resistance. Additionally, the aspect can now use the options in the "Mythic Actions" section for 1 hour.

Innate Spellcasting (3/Day). Tiamat's innate spellcasting ability is Charisma (spell save DC 26, +18 to hit with spell attacks). She can innately cast the following spells:

1/day each: gate, wish, feeblemind, teleport, dominate monster, antimagic field

Legendary Resistance (5/Day). If Tiamat fails a saving throw, she can choose to succeed instead.

Limited Magic Immunity. Unless she wishes to be affected, Tiamat is immune to spells of 6th level or lower. She has advantage on saving throws against all other spells and magical effects.

Magic Weapons. Tiamat’s weapon attacks are magical.

Multiple Heads. Tiamat can take one reaction per turn, rather than only one per round. She also has advantage on saving throws against being knocked unconscious. If she fails a saving throw against an effect that would stun a creature, one of her unspent legendary actions is spent.

Regeneration. Tiamat regains 100 hit points at the start of her turn. If she's been damaged by a Tooth of the Dragon since the end of her last turn, she instead regenerates 30 hit points at the start of her turn.

Multiattack. Tiamat can use her Frightful Presence. She then makes four attacks: one with her bite, two with her claws and one with her tail.

Bite. Melee Weapon Attack: +19 to hit, reach 20 ft., one target. Hit: 32 (4d10 + 10) slashing damage plus 14 (4d6) force damage.

Claw. Melee Weapon Attack: +19 to hit, reach 15 ft., one target. Hit: 24 (4d6 + 10) slashing damage. If the target is a Huge or smaller creature, it is grappled (escape DC 27) and is restrained until this grapple ends. Tiamat can have only one creature grappled this way at a time.

Tail. Melee Weapon Attack: +19 to hit, reach 25 ft., one target. Hit: 28 (4d8 + 10) piercing damage. If the target is a creature, it must succeed on a DC 27 Strength saving throw or be knocked 10ft back and prone.

Frightful Presence. Each creature of Tiamat’s choice that is within 240 feet of Tiamat and aware of her must succeed on a DC 26 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 Tiamat’s Frightful Presence for the next 24 hours.

Divine Word (3/Day). Tiamat casts divine word, using the same spellcasting ability as Spellcasting.

Protective Magic (3/Day). Tiamat casts counterspell or shield in response to the spell’s trigger, using the same spellcasting ability as Spellcasting.

Tiamat can take 5 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. Tiamat regains spent legendary actions at the start of her turn.

Move. Tiamat moves up to her speed.

Bite. Melee Weapon Attack: +19 to hit, reach 20 ft., one target. Hit: 32 (4d10 + 10) slashing damage plus 14 (4d6) force damage.

Black Dragon: Acid Breath (Costs 2 Actions). Tiamat exhales acid in a 120-foot line that is 10 feet wide. Each creature in that line must make a DC 27 Dexterity saving throw, taking 67 (15d8) acid damage on a failed save, or half as much damage on a successful one.

Blue Dragon: Lightning Breath (Costs 2 Actions). Tiamat exhales lightning in a 120-foot line that is 10 feet wide. Each creature in that line must make a DC 27 Dexterity saving throw, taking 88 (16d10) lightning damage on a failed save, or half as much damage on a successful one.

Green Dragon: Poison Breath (Costs 2 Actions). Tiamat exhales poisonous gas in a 90-foot cone. Each creature in that area must make a DC 27 Constitution saving throw, taking 77 (22d6) poison damage on a failed save, or half as much damage on a successful one.

Red Dragon: Fire Breath (Costs 2 Actions). Tiamat exhales fire in a 90-foot cone. Each creature in that area must make a DC 27 Dexterity saving throw, taking 91 (26d6) fire damage on a failed save, or half as much damage on a successful one.

White Dragon: Cold Breath (Costs 2 Actions). Tiamat exhales an icy blast in a 90-foot cone. Each creature in that area must make a DC 27 Dexterity saving throw, taking 72 (16d8) cold damage on a failed save, or half as much damage on a successful one.

If Tiamat's Chromatic Wrath trait has activated in the last hour, she can use the options below as legendary actions.

Cast a Spell. Tiamat casts a spell.

Multiattack. Tiamat uses Multiattack.

Breath Weapons (2 Actions). Tiamat uses two breath weapons at the same time.