Tiamat(Human Form)

Compressed and Condensed. When in combat, regenerate 5D12 Hit Points every turn. When outside of combat, negate any damage under 60.

This Shell Is Only For Show. If she is brought down to 0 Hit Points, she loses her form and is returned to her True Form.

Spellcasting. The Humanoid form of Tiamat is a 20th level Sorcerer and a 18th Level Wizard. Its spellcasting ability is Charisma. Her spell save DC 23. +12 to hit with spell attacks. The Humanoid Tiamat has the following spells prepared:
At will: Ray of Frost, Thunderclap, Firebolt, Dragon’s Breath, Shield

6/day each: Counterspell, Dominate Person, Fire Storm, Project Image, 

3/day each: True Resurrection, Meteor Swarm, 

Tiamat may take up to 4 Actions and 2 Bonus Actions on her turn.

A Dragon, This Queen. Once at the beginning of her turn in battle, Tiamat can force one target to make a Wisdom Saving throw, The DC is 23. On Failure, the target has disadvantage against Tiamat for attacks and Saving Throws until the end of their turn.

Multiattack. Tiamat makes three Regal Claw and/or Royal Smite Attacks

Regal Claw. Melee Attack: +11 to hit, reach 5 ft., 1 target. Hit: 35 (5D12+10) Slashing damage. Tiamat makes one of her hands grow five draconic talons from her fingertips and tears into her opponent with them. 

Royal Smite. Melee Attack: +9 to hit, reach 5ft., 1 Target Hit: 45 (6D10+15) Bludgeoning damage. Tiamat covers her body with draconic scales and unleashes a combo of attacks to inflict grievous injury. Force a DC19 Constitution Saving Throw. On failure, the target takes an additional 4D6 Necrotic damage and is knocked Prone.

Dragon Wreath. Ranged Attack: +12 to hit, range 15/150 ft., 1 target. Hit: 45 (4D10 + 15)Fire, Cold, Thunder, Acid or Poison damage. Creating a ring of massive elemental energy, Tiamat wraps her hand with it and flings it toward one target.

Rise Once More. As a bonus action, Tiamat can call upon her fallen allies and bring them back from the dead with half of their original Hit Points. Their fatal wounds are healed and any missing limbs/extremities/organs are restored. This is only usable twice per long rest.

Risk is Reward, Child. Once per turn, she may restore 6D10 of her or an ally's Hit points or inflict 6D10 Necrotic damage to a single target as an attack. If attacking, the target must make a Constitution Saving throw(DC23). On failure, the target takes full damage and is knocked prone. On Success, the damage is done instead to Tiamat, doubled.

Tiamat may take up to three reactions per round.

Regal Draconic Fervor. When Tiamat is hit, she may use her reaction to reduce damage by 6D12. Should the damage be reduced to zero, inflict the result as rebounded Force damage to the attacker instead.

Tiamat may take 6 Legendary Actions per turn. Each Legendary Action is only usable once, beginning at different levels of each battle. She regains all her Legendary Actions at the beginning of her turn.

Tiamat, Queen of Dragons (Costs 4 Actions, 1/Day) If Tiamat would be dropped to 0 Hit Points, she instead is restored to 60 Hit points and Immune to Damage until her next turn.

Starting when 50 or more damage is done:

Translocation Damage (Costs 2 Actions, 2/Turn). If Tiamat takes Damage use this action to make a Constitution Saving Throw(DC20). On Success, move the damage to another target and split it in half. On failure, the damage is not relocated.

Starting when 100 or more damage is done:

A Queen's Will (Costs 1 Action, 4/Day) If Tiamat fails a Saving Throw, she may succeed instead.

When Tiamat’s Hit Points reach 55, she will gain her Mythic Actions. Her eyes will blaze a bright red and her horned crown will begin to glow as if to be on fire. She will sprout two wings on her back, black in scale color. For the first turn of activation, Tiamat will gain 6 Mythic Actions. Each turn, at the beginning of her turn, she will gain 2D4+3 Mythic Actions and lose her remaining Mythic Actions from her previous turn(if any). These actions can be expended as a Reaction on her or her opponents turns, an Action and/or a Bonus Action. Tiamat cannot use the same Mythic Action more than once per turn.

Heavy Lay The Crown (Costs 4 Actions). As a Reaction, when damage is inflicted upon Tiamat, recover 3D12 Hit Points.

My Claw, Your Throat (Costs 3 Actions). Melee Attack: +9 to hit, range 15 ft., 1 Target. Hit: 30 (10D6) Poison Damage. Tiamat manifests her draconic claw and coats it with life sapping magic. She swings with great speed. On hit, Tiamat gains Hit Points equal to half of the damage inflicted.

Aspect of the Dragon Queen: Avernus Call (Costs 6 Actions). Tiamat let’s out a deafening roar, forcing any enemy(of her choice) who hears it to make a Wisdom Saving Throw(DC23). On failure, the targets will take 10D10 Psychic Damage as being afflicted with Fear. On success, targets will take half the damage and resist the adverse effects.