Ancient Grey Dragon

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

Reincarnated Aura. The dragon gains a special aura based on it's breath's damage type in it's previous life. Each creature of the dragon's choice that is within 120 feet gains resistance to the damage type of the dragon's previous breath. Additionally each damage type has an additional effect.
Fire:  Each creature of the dragon's choice that is within 120 feet radiates fire, dealing 1d10 fire damage to every creature in a 10ft radius at the start of their turn.
Cold: Each creature of the dragon's choice that is within 120 feet is cold to the touch. On their turn they can, as a bonus action, perform an unarmed attack that deals 1d12 cold damage and the target must succeed on a DC 18 Constitution saving throw or become frozen for one round. Additionally water beneath their body becomes a platform of ice that can be walked on.
Poison: Each creature of the dragon's choice that is within 120 feet cannot be poisoned, or is unaffected by poison already in their system while in the aura.
Acid: Each creature of the dragon's choice that is within 120 feet can generate acid from their body, powerful enough to melt through rock and metal. On their turn they can, as a bonus action, perform an unarmed attack that either deals 2d8 acid damage or destroys a target's weapon as long as it is not adamantine.
Lightning: Each creature of the dragon's choice that is within 120 feet produces large amounts of static electricity. Any creature that touches an affected creature physically or through a metal object, is electrocuted and takes 1d10 lightning damage.
Psychic: Each creature of the dragon's choice that is within 120 feet is cannot be charmed, frightened, or stunned by an effect or spell that requires a Intelligence, Wisdom, or Charisma Saving throw. However any creature already affected before entering the aura does not have those condtions removed.

Multiattack. The dragon 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 15 ft., one target. Hit: 19 (2d10 + 8) piercing damage.

Claw. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 15 (2d6 + 8) slashing damage.

Tail. Melee Weapon Attack: +14 to hit, reach 20 ft., one target. Hit: 17 (2d8 + 8) 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 18 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 Weapons (Recharge 5–6). The dragon uses one of the following breath weapons:

Death Breath. The dragon exhales magical shadowflame in an 90-foot cone. Each creature of the dragon's choice in that area must make a DC 21 Dexterity saving throw, taking 56 (16d6) necrotic damage on a failed save, or half as much damage on a successful one.

Life Breath. The dragon exhales magical mist in a 90-foot cone. Each creature of the dragon's choice in that area heals 56 (16d6) hit points and cannot die for 10 minutes, but they still can be incapicated after falling to 0 hit points.

Change Shape. The dragon magically polymorphs into a humanoid or beast that has a challenge rating no higher than its own, or back into its true form. It reverts to its true form if it dies. Any equipment it is wearing or carrying is absorbed or borne by the new form (the dragon's choice).

In a new form, the dragon retains its alignment, hit points, Hit Dice, ability to speak, proficiencies, Legendary Resistance, lair actions, and Intelligence, Wisdom, and Charisma scores, as well as this action. Its statistics and capabilities are otherwise replaced by those of the new form, except any class features or legendary actions of that form.

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 15 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.