Ancient Shadow Astral Dragon

Living Shadow. While in dim light or darkness, the dragon has resistance to damage that isn’t force, psychic, or radiant.

Shadow Stealth. While in dim light or darkness, the dragon can take the Hide action as a bonus action.

Sunlight Sensitivity. While in sunlight, the dragon has disadvantage on attack rolls, as well as on Wisdom (Perception) checks that rely on sight.

Immutable Form. The golem is immune to any spell or effect that would alter its form.

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

Detect Extraplanar: An astral dragon while in the astral plane has advantage in Wisdom (Perception) checks to detect creatures not native to that plane.

Plain Shifter: The astral dragon can plane shift at will but every time it does it has 1% to contract a disease called astrolomitis that prevents the dragon to plane shift. If the dragon contracts the disease in any plane other than the Astral Plane it is trapped forever in that plane until the disease is removed, which can only happen if healed by a wish.

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: +16 to hit, reach 15 ft., one target. Hit: 19 (2d10 + 8) piercing damage plus 9 (2d8) necrotic damage.

Claw. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 15 (2d6 + 8) slashing damage.

Tail. Melee Weapon Attack: +16 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 19 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.

Shadow Breath (Recharge 5–6). The dragon exhales shadowy fire in a 90-foot line that is 10 feet wide. Each creature in that line must make a DC 22 Dexterity saving throw, taking 67 (15d8) necrotic damage on a failed save, or half as much damage on a successful one. A humanoid reduced to 0 hit points by this damage dies, and an undead shadow rises from its corpse and acts immediately after the dragon in the initiative count. The shadow is under the dragon’s control.

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.

Tail Attack. The dragon makes a tail attack.

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.

Astral Knowledge (Costs 2 Actions). The astral dragon casts a spell of 3rd level or lower.

Astral Gate (Costs 2 Actions).  The astral dragon teleports to a location it can see within 60 ft.