Astral Wyrm

Amphibious. The dragon can breathe air and water.

Astral nature. The dragon does not require air, food, beverage, or rest to survive.

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

Astral Awakening (Recharges ofter a Short or Long Rest). If
the Wyrm would be reduced to 0 hit points, its current
hit point total instead resets to 596 hit points, it recharges its
Breath Weapons, and it regains any expended uses of Legendary
Resistance. Additionally, the Wyrm can now use the options in the "Mythic Actions" section for 1 hour.

Multiattack. The dragon can use its Frightful Presence. It then can make up to five attacks of its choice.

Bite. Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit: 21 (2d10+10) piercing damage, 7 (2d6+2) radiant damage, and 7 (2d6+2) necrotic damage.

Claw. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 17 (4d6+10) slashing damage and 12 (4d6) force damage. If the attack hits a large or smaller creature the dragon can grab the creature. A grabbed creature takes no damage and is grappled (DC 26 strength save to break free). The grappled creature is restrained until the grapple ends. The dragon can take an action to throw a grappled creature up to 200ft. A thorn creature must make a DC 21 dexterity save to land safely. On failure the creature takes 18 (3d10+4) bludgeoning damage.

Tail. Melee Weapon Attack: +17 to hit, reach 40 ft., all targets in range. Hit: 19 (2d8 + 10) bludgeoning damage, 9 (2d8+2) force damage and all targets are knocked prone.

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 27 Wisdom saving throw or become frightened for 2 minutes. 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 12 hours.

Breath Weapons (Recharge 4–6).
The dragon uses one of the following breath weapons.

Solar Beam. The dragon exhales pure white energy in a 300 ft line. Each creature in that line takes 55 (11d10) fire damage and 55 (11d10). A creature Immune to one of these damages still takes damage from the other. An creature hit by the solar beam is scorched and loses 5 hp each round. This effect lasts until the creature takes cold damage or comes into contact with water or ice.

Lunar Ray. The dragon exhales black energy that semestre to absorb light around it in a 300 ft line. Each creature in that line take 55 (11d10) cold damage and 55 (11d10) necrotic damage. Each creature hit by this attack become petrified in ice and must make a DC 22 strength save to break free.

Sapping Breath. The dragon exhales gas in a 90-foot cone. Each creature in that area must succeed on a DC 27 Constitution saving throw or be knocked unconscious 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.

Change Shape. The dragon magically polymorphs into a creatare 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 All of its game stats, as well as this action. Its attacks 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 6 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.

Astral Shard. The dragon fires an unidentifiable crystal at a target. The target takes 11 (3d6+4) piercing damage and 11 (3d6+4) psychic damage. The crystal is worth 12000 gp but only because it’s is so rare and entrancing.

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 30 feet of the dragon must succeed on a DC 25 Dexterity saving throw or take 17 (4d6 + 10) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.

If the greatwyrm's Metallic Awakening trait has activated in the
last hour, it can use the options below as legendary actions.

Bite. The greatwyrm makes one Bite attack. 

Shattering Roar (Costs 2 Actions). The Wyrm unleashes
a magical roar. Each creature in a 120-foot-radius sphere
centered on the Wyrm must succeed on a DC 26 Consti-
tution saving throw or take 19 (3d12 + 2) thunder damage and be
incapacitated until the end of its next turn

Phantasmal armor (Costs 2 Actions). Purple energy temporarily
surrounds the GreatWyrm, and it gains 50 temporary hit points until the start of its next turn. Until all these temporary hit points are gone, any creature that touches the Wyrm or hits it with a melee attack takes 26 (4d12+4) psychic damage.