Legendary Resistance (3/Day).  If the dragon fails a saving throw, it can choose to succeed instead.

Shifting scale. At the start of the dragon turn roll (1d10) it sheds its current scales to reveal a new colored set underneath. Its damage immunity changes to the new scale type. 
1. Red, fire
2. White, cold
3. Green, poison
4. Blue, lightning
5. Black, acid
6. Sapphire, thunder
7. Amethyst, force
8. Crystal, radiant
9. Topaz, necrotic
10. Emerald, psychic 

Multiattack. The dragon can use its Chaotic Presence. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +14 to hit, reach 5 ft., 1 target. Hit: 19 (2d10 + 8) piercing damage plus 7 (2d6) element damage. 

Claw. Melee Weapon Attack: +14 to hit, reach 5 ft., 1 target. Hit: 15 (2d6 + 8) slashing damage. 

Tail. Melee Weapon Attack: +14 to hit, reach 15 ft., 1 target. Hit: 17 (2d8 + 8) bludgeoning damage. 

Chaotic 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 19 Wisdom saving throw or be affected by one of its chaotic presence options. Roll (1d6) to determine which effect is used this turn.
1) Frightening: Creatures that fail the save become frightened until the end of the dragon's next turn. 
2) Blinding: Creatures that fail the save become blinded until the end of the dragon's next turn. 
3) Deafening: Creatures that fail the save become deafened until the end of the dragon's next turn. 
4) Floating: Creatures that fail the save become affected by the levitate until the end of the dragon's next turn. 
5) Confusing: Creatures that fail the save become affected by the confusion until the end of the dragon's next turn.
6) Wild: Creatures that fail the save must roll on A wild magic table.

Elemental breath (recharge 5-6). The dragon exhales an elemental breath in a 60-foot cone. Roll (1d8) to determine what type of damage the breath deals. Each creature in that area must make a DC 21 Dexterity saving throw, taking 64 (16d6) element damage on a failed save, or half as much damage on a successful one.
1) Fire
2) Cold
3) Poison
4) Lightning
5) Acid
6) Thunder
7) Necrotic
8) Radiant

The dragon can take 3 legendary actions, choosing from the options below. Only one legendary action 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 (Costs 1 Action). The dragon makes a Wisdom perception

Tail attack (Costs 1 Action). The dragon makes a tail attack.

Legendary Action Name (Costs 2 Actions). The dragon beats its wings. Each creature within 10 feet of the dragon must succeed on a DC 22 Dexterity saving throw or take 15 (2d6 + 8) bludgeoning damage and be knocked prone The dragon can then fly up to half its flying speed.