Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.
Godess: If Tiamat drops to 0 hit points, she reforms in the Abyss instantly with full health.
Multiattack (Recharge 6) The dragon can use its Frightful Presence. It then makes seven attacks: five with its bite and two with its claws.
Bite. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 21 (2d10 + 10) piercing damage plus 28 (8d6) (choose: fire, lightning, acid, poison or cold) damage.
Claw. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 29 (4d6 + 10) slashing damage.
Tail. Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit: 23 (3d8 + 10) bludgeoning damage.
Frightful Presence. Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 21 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 the dragon's Frightful Presence for the next 24 hours.
Breath Weapon (Recharge 5–6). The dragon exhales energy in a 90-foot cone. Each creature in that area must make a DC 26 Dexterity saving throw, taking 107 (30d6) (choose: fire, lightning, acid, poison or cold) damage on a failed save, or half as much damage on a successful one.
The dragon 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. The dragon regains spent legendary actions at the start of its turn.
Detect. The dragon makes a Wisdom (Perception) check.
Tail Attack. The dragon makes a tail attack.
Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 15 feet of the dragon must succeed on a DC 25 Dexterity saving throw or take 17 (2d6 + 10) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.
Tiamat has five heads, each of which correspond to a type of evil chromatic dragon.