Astral Plane Blend As an action, the Dragon can blend with its surroundings to stealth advantage.

Amphibious The dragon can breathe air and water

Change Shape. The Astral Dragon magically polymorphs into a humanoid that has the same stats and abilities as its own, or back into its true self. It reverts to its true form if it dies. Any equipment it is wearing or carrying is absorbed or borne by the new form. 

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

Spellcasting. The Ancient Astral Dragon is a 18th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 21, +15 to hit with spell attacks). The Ancient Astral Dragon has the following spells to choose from, which it may cast without material components:

Cantrips (All at will*): Dancing Lights, Mage Hand, Magic Stone, Spare the Dying, Fire Bolt, Shocking Grasp, Acid Splash, Poison Spray

1st level (5 slots): Detect Magic*, Detect Evil and Good*, Bane, Command, Comprehend Languages, Thunderwave

2nd level (4 slots): Blur*, Alter Self*, Misty Step*, Darkness, Blindness/Deafness, Invisibility, Scorching Ray, See Invisibility, Zone of Truth

3rd level (4 slots): Spirit Guardians*, Tongues*, Beacon of Hope, Counterspell,  Dispel Magic, Remove Curse, Slow, 

4th level (4 slots): Dimension Door*, Banishment**, Confusion, Greater Invisibility

5th level (3 slots): Contact Other Plane*, Dreams*, Dominate Person, Greater Restoration, Mass Cure Wounds, Wall of Force

6th level (3 slots): Forbiddance*, Guards and Wards, Heal, 

7th level (2 slots): Plane Shift*, Prismatic Spray*, Reverse Gravity*, Regenerate, Resurrection, 

8th level (2 slot): Demiplane*, Antimagic Field, Power Word Stun, Sunburst 

9th level (1 slot): Astral Projection*, Gate*, Imprisonment**, Mass Heal, Meteor Swarm, Prismatic Wall, Time Stop, True Resurrection, Wish

Multiattack. The dragon can make three attacks during its turn.

Bite. Melee Weapon Attack: +14 to hit, reach 30 ft., one target. Hit: 19 (2d10 + 8) piercing damage. If the attack roll for this Bite is a natural 20, and the target is in Astral Plane via the Astral Projection spell or similar magic, the silver cord connecting the body of the creature to its soul is instantly destroyed - killing the physical body but keeping the astral body of the creature stuck in the astral plane forever.

Claw. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 15 (2d8 + 8) slashing damage.

Tail. Melee Weapon Attack: +14 to hit, reach 60 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 5 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 24 astral hours.

Stupefying Breath (Recharge 5-6). The dragon exhales a beam of darkness psychic energy in a 150-foot long, 15-foot wide beam. Each creature in that area must succeed on a DC 21 Intelligence saving throw or become mentally drained. If the creature succeeds it gains immunity to it for 24 astral hours. For the next 1 astral minute, the creature's Intelligence, Wisdom and Charisma scores become 1. The creature can't cast spells, activate magic items, understand language, or communicate in any intelligible way. The creature can, however, identify its friends, follow them*, and even protect them. A creature drained in this way may repeat the saving throw at the end of each of their turns to end the effect on themselves. 

*If on the Astral Plane, a creature affected by this breathe would also be unable to move, due to the nature of movement on this plane.

Prismatic Breath (Recharge 5-6): The dragon rears back and exhales a blast of prismatic color in a 100-foot line that is 10 feet wide. Each creature must make a (DC 20) DEX saving throw, taking 110 (19d10) Prismatic Damage (Fire, Ice, Lightning, Necrotic, Force, Radiant, Acid, Poison) (roll 1d8 per player to see which damage type hits which player)

Attack of opportunity, counterspell 

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 (with advantage if on the Astral Plane).

Cantrip. The dragon uses a Cantrip from it's spell list.

Psychic Wind (Costs 2 Actions). The dragon causes a blast of psychic wind to emanate from itself. Each creature within 30 feet of the dragon must succeed on a DC 21 Intelligence saving throw or take 18 (3d10) psychic damage and become stunned for 1 minute. A creature stunned in this way may repeat the saving throw at the end of each of their turns to end the effect on themselves.

Legendary Resistances (3/day). If the dragon fails a saving throw, it can succeed it instead

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

Breath Move (2 Actions). The dragon can move its head 10 ft left to right when using it's breath attack.

Spiral Attack (3 Actions). The dragon coils itself into a spring like shape, then spirals out at extreme speeds. Each creature within 30 ft of the dragon must succeed on a (DC 19) dexterity saving throw, on a failed save, the creature can be repulsed back for 60 ft, suffering 5d10 force damage and are knocked prone, on a successful save the creature remains and cannot be thrown for 10 minutes.