Wrath, Offspring of Tiamat

Molten Trap. 

All attacks that deal fire damage make a DC 22 Dexterity Saving Throw or  be stuck in molten rock, needing a DC 19 Strength  Saving throw, or be unable to take any action, or bonus action that requires the use of limb, such as fireball, a longsword, or a long bow. The head is the only part of the body that isn't connected. You may repeat the saving throw until you escape. If you are still stuck on the third turn, the molten rock hardens, the the DC for the Strength check to 22.

Spellcasting. Wrath is a 9th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 21, + 7 to hit with spell attacks). Wrath has following spells, requiring no spell components
Cantrips (at will): Eldritch Blast,  Message, 

8th Level(5 Slots): Earthquake, Incendiary Cloud, Sunburst, Fireball

9th level (1 slots): Storm of Vengeance, Mass Heal

Multiattack. Wrath may make a Volcanic Bite with each of it's three heads, two Molten Claw attacks, or a Molten Breath attack, with each of her three heads.

Volcanic Bite. Melee Weapon Attack: +17 to hit, reach 15ft ft., one target. Hit: 31 (4d 10+15) piercing damage damage plus 20(3d6+11) fire damage. 

Molten Breath (Recharge 5–6). The dragon exhales fire in a 90-foot cone. Each creature in that area must make a DC 24 Dexterity saving throw, taking 91 (15d6) fire damage on a failed save, or half as much damage on a successful one. The target is takes 2d6 fire damage every turn, until the target makes a DC 15 Constitution saving throw.

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.

Blazing Tail. Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit: 19 (2d8 + 10) bludgeoning damage and hit: 12 (1d8 + 8) fire damage.

Molten Claw. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 17 (2d10 + 13) slashing damage, and (3d6+5) fire damage.

The dragon can take 3 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.

Blazing Tail Attack. The dragon makes a tail attack.

Eruption (Costs 3 Actions). Wrath erupts molten meteors from every head, hitting every creature with 100 feet of her. Each creature has to make a DC 23 Dexterity Saving Throw, Taking (15d6+13) fire damage, and (4d10+4) bludgeoning damage on a failed save, and have as much on a successful one.