Chromillia, Incarnation of Tiamat

Amphibious. Chromillia can breathe air and water.

Ice Walk. Chromillia can move across and climb icy surfaces without needing to make an ability check. Additionally, difficult terrain composed of ice or snow doesn't cost it extra movement.

Regeneration. Chromillia regains 10 hit points at the start of her turn.

Color Change. Chromillia can become resistant to either Fire, Acid, Cold, Lightning and Poison Damage. (Identical to Deathwalker's Ward from Critical role). To swap elemental resistances requires a bonus action. Chromillia is also resistant to all Piercing, Bludgeoning and Slashing damage from non-magical weapons. 

Telepathic Bond. Chromillia can contact Tiamat (and Vice Versa) regardless of her location, as long as they are both alive and conscious.

Human Form. Chromillia may shape change into a human for any intents and purposes. (Blending in, size convenience, etc) 

Legendary Resistance (3/Day). If Chromillia fails a saving throw, she can choose to succeed instead.

Multiattack. Chromillia can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 19 (2d10 + 8) piercing damage plus 7 (2d6) elemental damage. (Element Determined by whatever Chromillia's active resistance is.)

Claw. Melee Weapon Attack: +14 to hit, reach 5 ft., one target. Hit: 15 (2d6 + 8) slashing damage.

Tail. Melee Weapon Attack: +14 to hit, reach 15 ft., one target. Hit: 17 (2d8 + 8) bludgeoning damage.

Frightful Presence. Each creature of Chromillia's choice that is within 120 feet of her and aware of it must succeed on a DC 16 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 Attacks

• At the start of her turn, Chromillia may roll 1d8. She regains a charge of a respective breath or spell attack based on the roll, as shown below. She has 1 charge of each breath attack and spell. 

Acid Breath (Recharge 3). The dragon exhales acid in a 60-­foot line that is 5 feet wide. Each creature in that line must make a DC 18 Dexterity saving throw, taking 54 (12d8) acid damage on a failed save, or half as much damage on a successful one.

Cold Breath (Recharge 4). The dragon exhales an icy blast in a 60-foot cone. Each creature in that area must make a DC 19 Constitution saving throw, taking 54 (12d8) cold damage on a failed save, or half as much damage on a successful one.

Fire Breath (Recharge 5). The dragon exhales fire in a 60-foot cone. Each creature in that area must make a DC 21 Dexterity saving throw, taking 63 (18d6) fire damage on a failed save, or half as much damage on a successful one.

Lightning Breath (Recharge 6). The dragon exhales lightning in a 90-­foot line that is 5 feet wide. Each creature in that line must make a DC 19 Dexterity saving throw, taking 66 (12d10) lightning damage on a failed save, or half as much damage on a successful one.

Poison Breath (Recharge 7). The dragon exhales poisonous gas in a 60-foot cone. Each creature in that area must make a DC 18 Constitution saving throw, taking 56 (16d6) poison damage on a failed save, or half as much damage on a successful one.

Prismatic Spray (Recharge 8). Chromillia may cast the spell Prismatic Spray

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.

Tail Attack. The dragon makes a tail attack.

Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 10 feet of the dragon must succeed on a DC 20 Dexterity saving throw or take 14 (2d6 + 7) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.