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

Magic Resistance. The dragon has advantage on saving throws against spells and other magical effects.

Astral Sight. The dragon can see clearly through the blur of the Astral Plane for up to 10 miles (16 km).

Detect Extraplanar. The dragon has advantage on checks made to detect creatures not native to the Astral Plane.

Innate Plane Shift. The dragon can cast Plane Shift at will without material components to any plane and location it is aware of. After reaching the new plane, roll a d100. On a 1 the dragon develops Astralomitis as it loses its innate plane shift.

Variant Spellcasting. The dragon is a 20th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 21, +13 to hit with spell attacks). It has the following spells prepared:

Cantrips (at will): mind sliver, mage hand, mending, minor illusion

1st level (4 slots): bane, gift of alacrity, magic missile, prestidigitation

2nd level (3 slots): blur, invisibility, heat metal, misty step, tasha's mind whip

3rd level (3 slots): counterspell, dispel magic, slow, lightning bolt, sending

4th level (3 slots): banishment, charm monster, phantasmal killer

5th level (3 slots): arcane hand, dispel evil and good, hold monster, telekinesis

6th level (2 slots): flesh to stone, disintegrate

7th level (2 slots): forcecage, prismatic spray

8th level (1 slot): reality break

9th level (1 slot): weird

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: +13 to hit, reach 10 ft., one target. Hit: 18 (2d10 + 7) piercing damage. If the attack is a critical hit and the target is a creature travelling through the Astral Plane by means of the astral projection spell, the dragon can cut the target’s silver cord instead of dealing damage, killing the creature instantly.

Claw. Melee Weapon Attack: +13 to hit, reach 5 ft., one target. Hit: 14 (2d6 + 7) slashing damage.

Tail. Melee Weapon Attack: +13 to hit, reach 20 ft., one target. Hit: 16 (2d8 + 7) 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 20 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 5–6). The dragon exhales a beam of psychic energy in a 200-foot long and 5-foot wide line. Each creature in the line must make a DC 20 Intelligence saving throw. On a failure, 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. The creature can reroll the saving throw (using its normal bonus to the saving throw) at the end of its next turn, ending the effect on a success.

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.

(Variant) Cantrip. The dragon casts a cantrip.

Detect. The dragon makes a Wisdom (Perception) check.

Tail Attack. The dragon makes a tail attack.

Dashing Charge (Costs 2 Actions). The dragon charges in a straight line up to half its flying speed. Any creature in its path must make a DC 21 Dexterity saving throw or take 14 (2d6 + 7) and be knocked prone.

Astral Guardian (Mythic Trait; Recharges after a Short or Long Rest). When the Astral Dragon is reduced to 0 hit points, it doesn’t die or fall unconscious. Instead, the surrounding energy of the Astral Plane comes to the dragon's aid, regaining 315 hit points. Additionally, the Astral Dragon becomes surrounded by a storm of psychic energy. If the Astral Dragon succeeds on saving throw against a spell, or a spell attack misses it, the Astral Dragon can choose another creature (including the spellcaster) it can see within 120 feet of it. The spell targets the chosen creature instead of the Astral Dragon. If the spell forced a saving throw, the chosen creature makes its own save. If the spell was an attack, the attack roll is rerolled against the chosen creature.

If the Astral Dragon's mythic trait is active, it can use the options below as legendary actions for 1 hour after using Astral Guardian:

(Varaint) Cast. The dragon casts a spell.

Dash. The dragon moves its full movement.

Quick Slash. The dragon makes two claw attacks.

Astral Pulse (Costs 3 Actions). The dragon emits a pulse of psychic energy in a 30-foot radius around itself. Any creature in the area must make a DC 20 Intelligence saving throw, taking 35 (10d6) psychic damage on a failure, or half as much damage on a success.