Damage Threshold. Tiamat has a damage threshold of 20. If a single creature does damage multiple times in a single turn, combine the damage to determine whether the threshold is exceeded.

Many Heads. Tiamat has five heads, each of which can only act once per round (of hers). At the start of each of her turns, roll 1d10 for which head takes that action (1-2: white; 3-4: black; 5-6: green; 7-8: blue; 9-10: red); other heads take legendary actions in any order. If the chosen head is disabled, use the next non-disabled head. An action with a color may only be taken with the head of the correct color.

Head Disabling.  For every 20% damage Tiamat takes (at 484, 363, 242, and 121 hp) one head (the head that most recently acted) becomes disabled. Disabled heads can only take the Recover and Regenerate legendary actions.

Head Sacrifice. If Tiamat fails a save, she may choose to succeed. If she does so, she is stunned until the end of her next turn, or she removes the condition.

Chromatic Separation. If Tiamat is reduced to zero hit points, she breaks apart; remove her from the map and spawn an Adult Black Dragon, Adult Blue Dragon, Adult Green Dragon, Adult Red Dragon, and Adult White Dragon. They are worth their normal XP value.

Bite. Melee Attack Roll: +19, reach 20 ft., one target. Hit: 23 (2d12 + 10) piercing damage plus 19 (3d12) damage of the same type as her breath.

Red Dragon Breath. Dexterity Saving Throw: DC 27, each creature in a 150' cone. Failure: 65 (10d12) fire damage, and burning for one minute; burning damage increased to 1d12. Success: half damage only.

Blue Dragon Breath. Dexterity Saving Throw: DC 27, each creature in a 300' line. Every time the breath passes through the space of a Small or larger object or creature or strikes a barrier it may change direction, out to a total length of 300', but no creature may be affected twice. Failure: 65 (10d12) lightning damage, stunned until the start of its next turn, and on its next turn may take either an action or a bonus action, but not both. Success: half damage only.

Green Dragon Breath. Constitution Saving Throw: DC 27, each creature in a 75' sphere centered within 75. Failure: 32 (5d12) poison damage. Success: half damage. Effect: creates a cloud that lasts for one minute or until Tiamat uses this attack again. Any creature that enters the cloud on a turn or ends its turn within the cloud suffers the breath effect. The cloud is heavily obscured.

Black Dragon Breath. Dexterity Saving Throw: DC 27, each creature in a 300' line. If the line hits a barrier, including the ground, it splashes outward in a sphere with a radius of 30'. Failure: 65 (10d12) acid damage, and -2 AC (armor may be repaired with mending, natural armor is repaired after a short rest). Success: half damage only.

White Dragon Breath. Constitution Saving Throw: DC 27, each creature in a 150' cone. Failure: 65 (10d12) cold damage, and restrained until the end of Tiamat's next turn. Success: half damage only.

Red Elemental Bolt. Ranged Attack Roll: +17, range 300 ft., one target. Hit: 39 (6d12) and the target is burning for one minute. Burning damage is increased to 1d12.

Blue Elemental Bolt. Ranged Attack Roll: +17, range 300 ft., one target. Hit: 39 (6d12) and the target is stunned until the start of its next turn, and on its next turn may take either an action or a bonus action, but not both.

Green Elemental Bolt. Ranged Attack Roll: +17, range 300 ft., one target. Hit: 39 (6d12) and the target is Poisoned until the end of its next turn.

Black Elemental Bolt. Ranged Attack Roll: +17, range 300 ft., one target. Hit: 39 (6d12) and the target's armor is corroded, reducing its AC by 2.

White Elemental Bolt. Ranged Attack Roll: +17, range 300 ft., one target. Hit: 39 (6d12) and the target is restrained until the end of Tiamat's next turn.

Spellcasting. Tiamat casts one of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 25, +17 to hit with spell attacks). Each head may concentrate separately.

• At Will: Detect Magic, Dispel Magic, Locate Object, Mage Hand
• 1x/day: Wish. If used for a purpose other than duplicating a spell of 8th level or lower, Tiamat is immediately banished to Avernus but suffers no other effects.
• 3x/day: Limited Wish, only usable to replicate a spell of 5th level or lower with a casting time of 1 action, bonus action, or reaction.
• Black Head: At Will: Darkness, Fear, 3x/day: Control Water.
• Blue Head: At Will: Invisibility, Thunder Step, 3x/day: Wind Wall.
• Green Head: At Will: Suggestion (3rd level version), 3x/day: Plant Growth
• Red Head: At Will: Command (3rd level version), 3x/day: Wall of Fire.
• White Head: At Will: Sleet Storm, 3x/day: Slow.

Spellcasting. Tiamat casts Misty Step, using Charisma as the spellcasting ability.

Legendary Action Uses: 4. Immediately after another creature's turn , the dragon can expend a use to take one of the following actions. The dragon regains all expended uses at the start of each its turns.

Bite. Tiamat uses Bite with her current head.

Elemental Bolt. Tiamat uses the Elemental Bolt of her current head.

Recover. Tiamat removes a condition or effect. Tiamat may use this ability even if unable to take actions, but must spend an additional legendary action to do so.

Regenerate. Tiamat recovers 61 (3d20+30) hit points. Tiamat may use this ability even if unable to take actions.

Spellcasting. Tiamat casts a spell other than wish with her current head.