Colossal. Tiamat is immune to forced movement, does not provoke opportunity attacks and cannot make opportunity attacks. 

In addition, whenever Tiamat makes a weapon attack, regardless of whether the attack hits or misses, each creature within 20 feet of the attack's target other than the target or Tiamat herself must succeed on a DC 27 Dexterity saving throw or takes half the attack's damage.

Discorporation. When Tiamat dies, her body is destroyed but her essence travels back to her domain in the Nine Hells, and she is unable to take physical form for a time.

Empowered Attacks. Tiamat's weapon attacks count as magical for the purpose of overcoming resistance and immunity to nonmagical attacks.

Immutable Form. Tiamat is immune to any spell or effect that would alter her form, except for her own Change Shape action.

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 that target her or include her in their area of effect. She has advantage on saving throws against all other spells and magical effects.

Multiple Heads. Tiamat has five heads, each resembling one of the chromatic Great Wyrms. These heads have the following properties:

• Tiamat has as many reactions each round as she has heads.
• Each head can concentrate on a seperate spell. Tiamat must roll concentration checks for each spell when she takes damage..
• Tiamat has advantage on saving throws against being incapacitated and against being knocked unconscious.
• If a character deals 200 or more damage to Tiamat in a single turn or a character wielding a vorpal sword rolls a natural 20 for an attack against Tiamat, one head of the character's choice is destroyed. The head regrows at the start of Tiamat's third turn after losing it.

Regeneration. TIamat regains 50 hit points at the start of her turn. If Tiamat takes radiant damage, this trait does not function at the start of her next turn. Tiamat only dies if she starts her turn with 0 hit points and does not regenerate.

Spellcasting. Tiamat is a 20th level spellcaster. Her spellcasting ability is Charisma (spell save DC 27, +19 to hit with spell attacks). She can cast the following spells, requiring no material components:

• Cantrips (at will): all
• 1st level (at will): bane, chromatic orb, command, detect magic, identify, inflict wounds, shield
• 2nd level (at will): blindness/deafness, dragon's breath, knock, misty step, ray of enfeeblement, suggestion
• 3rd level (at will): bestow curse, dispel magic, incite greed, remove curse, spirit guardians, vampiric touch
• 4th level (4 slots): banishment, blight, death ward, elemental bane, leomund's secret chest
• 5th level (3 slots): antilife shell, circle of power, contagion, legend lore, scrying
• 6th level (3 slots): circle of death, disintegrate, mass suggestion, scatter
• 7th level (3 slots): divine word, finger of death, plane shift (self only), prismatic spray
• 8th level (2 slots): earthquake, feeblemind, illusory dragon
• 9th level (2 slots): gate, power word kill, wish

Treasure Sense. TIamat knows the location of all magic items and of accumulations of mundane treasure within 5 miles of her.

Multiattack.Tiamat can use her World-Ending Presence, and if she was flying at the start of her turn, she can use her Crushing Landing. She then makes two attacks with her claws and one with her tail stinger and can make a bite attack with each head, but she can't bite the same target more than once.
For one head however, she can forgo the bite attack to be able to use its breath weapon as a legendary action.

Bite (Red). Melee Weapon Attack: +19 to hit, reach 35 ft., one target. Hit: 23 (2d12 + 10) piercing damage plus 42 (12d6) fire damage and Tiamat can grapple the target as part of that attack.

Bite (Blue). Melee Weapon Attack: +19 to hit, reach 35 ft., one target. Hit: 23 (2d12 + 10) piercing damage plus 16 (3d10) lightning damage and Tiamat can grapple the target as part of that attack. Tiamat has advantage on the attack roll if she targets a creature wearing armor made of metal with this attack.

Bite (Green). Melee Weapon Attack: +19 to hit, reach 35 ft., one target. Hit: 23 (2d12 + 10) piercing damage plus 21 (6d6) poison damage, the target must succeed on a DC 27 Constitution saving throw or be poisoned until the end of its next turn and Tiamat can grapple the target as part of that attack.

Bite (Black). Melee Weapon Attack: +19 to hit, reach 35 ft., one target. Hit: 23 (2d12 + 10) piercing damage plus 14 (3d8) acid damage and Tiamat can grapple the target as part of that attack. If the target wears nonmagical armor or wields a nonmagical shield, the armor or shield corrodes and is destroyed.

Bite (White). Melee Weapon Attack: +19 to hit, reach 35 ft., one target. Hit: 23 (2d12 + 10) piercing damage plus 14 (3d8) cold damage, the target must succeed on a DC 27 constitution saving throw or be encased in ice until the end of its next turn or until someone uses an action to break the ice off it. While the target is encased in ice, it has its movement speed reduced to 0 until the end of its next turn. Tiamat can grapple the target as part of that attack.

Claw. Melee Weapon Attack: +19 to hit, reach 40 ft., one target. Hit: 32 (4d10 + 10) slashing damage plus 27 (6d8) necrotic damage. If the target is a creature, it must succeed on a DC 27 strength saving throw or be flung up to 3d6 × 10 feet away from the dragon, landing prone.

Tail Stinger. Melee Weapon Attack: +19 to hit, reach 50 ft., one target. Hit: 36 (4d12 + 10) piercing damage plus 42 (12d6) poison damage and the target must make a DC 27 Constitution saving throw or be poisoned until the end of its next turn. While poisoned in this way, a creature is under the effect of a slow spell.

World-Ending Presence. Each creature of Tiamat's choice that is within 300 feet of Tiamat and aware of her must succeed on a DC 27 Wisdom saving throw or drop whatever it was holding and become frightened for 1 minute. A frightened creature must use the dash action on each of its turns, moving as far away from Tiamat as it can, using the safest possible route. If a creature ends its turn in a location where it does not have line of sight to Tiamat, it can repeat the saving throw, 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 World-Ending Presence for the next 24 hours.

Crushing Landing. If Tiamat is flying, she can attempt to land forcefully in a space of her choice, even if that space is occupied by other creatures. When she does so, her movement speed is reduced to 0 until the end of the turn, and each creature in Tiamat's space or within 60 feet of her, other than Tiamat herself, must make a DC 27 Dexterity saving throw or take 39 (6d12) bludgeoning damage and be knocked prone.

Breath of Doom (1/day). Tiamat can only use this breath weapon when she has all five heads and is not grappling anyone with her bite attacks and has not used a breath weapon since the end of her last turn.    
Tiamat combines the breaths of all her heads to exhale necrotic energy in a 500-foot cone. Each creature in that area must make a DC 27 Constitution saving throw, taking 180 (40d8) necrotic damage on a failed save, or half as much damage on a successful one. A creature reduced to 0 HP by this breath weapon dies.

In addition, from the breath's energy, five draconic spirits manifest, one of each color, in random locations within the area affected by the breath. These spirits the the forms of young dragons of the respective color, are under Tiamat's control and initiative is rolled separately for them. If they aren't killed, the spirits disappear after one hour.

Once she has used this breath weapon, she can't use any other breath weapon until the end of her next turn.

Change Shape. Tiamat magically polymorphs into her alternate form, which is that of the Dark Lady - a black-haired and black-eyed female human sorcerer wearing dark robes, or back into her true form. She reverts to her true form if she dies. Any equipment she is wearing or carrying is absorbed or borne by the new form (Tiamat's choice).

In her alternate form, Tiamat retains her alignment, hit points, hit dice, ability to speak, proficiencies, legendary resistance, lair actions, limited magic immunity, spellcasting and Intelligence, Wisdom, and Charisma scores, as well as this action. Her statistics and capabilities are otherwise replaced by those of her alternate form, except any class features or legendary actions of that form.

Quicken Spell (5/day). Tiamat casts one of her spells of 6th level or lower with a casting time of 1 action.

Divine Denial. When a creature within 300 feet of Tiamat that she can see casts a spell or targets Tiamat with an attack, she can use a reaction to force it to make a DC 27 Wisdom saving throw. On a failure, the spell or attack fails. Creatures immune to the charmed are immune to this effect. Clerics of good-aligned deities can use their Divine Intervention to prevent Tiamat from using this feature for 24 hours.

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.

Detect. Tiamat makes a Wisdom (Perception) check.

Tail Attack. Tiamat makes an attack with her tail stinger.

Wing Attack. Tiamat beats her wings. Each creature within 60 feet of Tiamat must succeed on a DC 27 Dexterity saving throw or take 28 (4d8 + 10) bludgeoning damage and be knocked prone. Tiamat can then fly up to half her flying speed.

Cast a Spell (Costs 2 Actions). Tiamat casts one of her spells of 6th level or lower.

Queen's Command (Costs 2 Actions).  Tiamat commands up to five chromatic dragons she can see within 300 feet of her to attack. Each dragon can use its reaction to move up to its speed without provoking opportunity attacks and to make one attack with its bite or claws.

Breath Weapon (Costs 3 Actions). Tiamat uses one of her breath weapons. She can only breath with a head that did not make a bite attack since the start of her last turn.

• Fire Breath (Red): Tiamat exhales fire in a 300-foot cone. Each creature in that area must make a DC 27 Dexterity saving throw, taking 119 (34d6)fire damage on a failed save, or half as much damage on a successful one. In addition, on a failure a creature gains a level of exhaustion due to the intense heat.
• Lightning Breath (Blue): Tiamat exhales lightning in a 500-foot line that is 20 feet wide. Each creature in that line must make a DC 27 Dexterity saving throw, taking 110 (20d10) lightning damage on a failed save, or half as much damage on a successful one. In addition, on a failed saving throw, a creature is paralyzed until the end of its next turn.
• Poison Breath (Green). Tiamat exhales poison in a 300-foot cone. Each creature in that area must make a DC 27 Constitution saving throw, taking 98 (28d6) poison damage on a failed save, or half as much damage on a successful save. In addition, a creature is poisoned until the end of its next turn on a failed save.
• Acid Breath (Black):  Tiamat exhales acid in a 500-foot line that is 20 feet wide. Each creature in that area must make a DC 25 Dexterity saving throw, taking 81 (18d8) acid damage on a failed save, or half as much damage on a successful one. In addition, a creature who fails the saving throw has disadvantage on attack rolls, ability checks and saving throws until the end of its next turn.
• Cold Breath (White): Tiamat exhales cold in a 300-feet cone. Each creature in that area must make a DC 27 Constitution saving throw, taking 90 (20d8)cold damage on a failed save, or half as much damage on a successful one. In addition, on a failed saving throw, a creature is encased in ice until the end of its next turn or until someone uses an action to break the ice off it. While encased in ice, a creature's movement speed is 0.