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

Spellcasting. The Ancient Astral Dragon is a 13th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 21, +13 to hit with spell attacks). The Ancient Astral Dragon has the following spells prepared, which it may cast without material components:

Cantrips (at will): dancing lights, mage hand, fire bolt

1st level (4 slots): bane, command, comprehend languages, detect magic

2nd level (3 slots): blur

3rd level (3 slots): dispel magic, nondetection

4th level (3 slots): banishment, confusion, freedom of movement

5th level (2 slots): synaptic static, wall of force

6th level (1 slot): arcane gate*, mental prison

7th level (1 slot):plane shift*, teleport

* The Ancient Astral Dragon can cast these spells at will, without expending a spell slot.

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. 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 it's soul is instantly destroyed - killing the target creature.

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.

Stupefying Breath (Recharge 6). The dragon exhales a beam of psychic energy in a 150-foot long, 10-foot wide line. Each creature in that area must succeed on a DC 21 Intelligence saving throw or become mentally drained. For the next 1 minute, the creature's Intelligence 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. The effect can also be ended by greater restoration, heal, or wish.

*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.

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).

Tail Attack. The dragon makes a tail attack.

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.