Brainstealer Dragon Greatwyrm

Awakening (Recharges after a rest). When the brainstealer dragon is reduced to 0 hit points, its current hit point total instead resets to 575, any expended legendary resistances are restored, it gains access to its mythic abilities, and the brainstealer dragon regains any expended spells other then feeblemind, time stop and wish. Additionally, award a party an additional 120,000 XP (275,000 XP total) for defeating the brainstealer dragon after it awakens.

Creature Sense. The brainstealer dragon is aware of the presence of creatures within 3 miles of it that have an Intelligence score. It knows the distance and direction of each creature, each one’s Intelligence score, and any magic or psionics that they may have. A creature protected by a mind blank spell, a nondetection spell, or similar magic can’t be perceived in this manner unless the brainstealer dragon succeeds on an intelligence save against the caster spell save DC.

Innate Spellcasting (Psionics). The brainstealer dragon's innate spellcasting ability is Intelligence (spell save DC 27, +19 to-hit with spell attacks). It can innately cast the following spells, requiring no components:

At will: detect thoughts, levitate, mind sliver (at max version)
5/day each: enemies abound, fear, Tasha's mind whip (5th level version)
3/day each: dominate monster, major image, mental prison, plane shift (self only)
1/day each: feeblemind

Innate Spellcasting (Aracne). The brainstealer dragon's innate spellcasting ability is Charisma (spell save DC 23, +15 to-hit with spell attacks). It can innately cast the following spells, requiring no components:

At will: hold person, shield
5/day each: counterspell
3/day each: banishment, greater invisibility
1/day each: globe of invunrability (8th level version), teleport, time stop, wish

Legendary Resistance (5/Day). If the brainstealer dragon fails a saving throw, it can choose to succeed instead.

Limited Magic Immunity. The braistealer dragon is not effected by spells of 5th level or lower enless it wishes to be.

Supreme Pyconics. The brainstealer dragon can concentrate on 2 different spells at once, and has advantage on saving throws that would end its concentration.

Uillithid Variant: 

• The brainstealer dragon's tentacle attacks range increases by 10ft (for a total of 40ft) and the damage increases to 52 (4d10 + 10) psychic damage; additionally, it can now grapple gargantuan-sized creatures and the Strength save DC becomes 22.
• Its extract brain attack gains the same increase in range and an additional 22 (4d10) damage.
• Its mind blast attack's recharge decreases to 3-4.
• Its psionic call legendary action rallies 2d8 + 2 Mind Flayers instead of 1d6 + 1.
• Its psychic shriek legendary action's damage increases by 18 (4d8) damage.
• Its hit point maximum increases by 103 (5d20 + 50).
• Its mass dominate monster mythic ability has a range of 100ft instead of 60.
• It can take 6 legendary actions and legendary resistances instead of 5.
• Any psychic damage that it deals ignores resistances and against immunities deals half damage as force damage (which also ignores resistances).
• Whenever it uses its legendary action to cast a spell it takes 3 legendary actions but can cast 2 spells at once, it may choose not to use this ability.
• If a part defeats a Uilithid variant award them an additional 75,000 XP.

Elder Brain Variant: 

• All of the buffs from the Uillithid variant.
• Its psionic call legendary action rallies all available Mind Flayers.
• Its hit point maximum increases by an additional 102 (5d20 + 50)
• Its mind blast attack gains a 200ft increase in range and an additional 22 (4d10) damage.
• Its psychic shriek legendary action's damage increases by another 18 (4d8) damage, however, it takes 3 legendary actions instead of 2.
• It can take 8 legendary actions and legendary resistances instead of 7.
• Any psychic damage that it deals ignores resistances and against immunities deals half damage as force damage (which also ignores resistances), and if you also have immunity to force damage screw you, you homebrew minmaxer you take half the force damage in radiant damage (which again ignores resistances) and if you have immunity to radiant damage, well I guess you beat me... Wait! Screw you again! You take half the necrotic damage in thundering damage (you bet this ignores resistances).
• It has optimized psionics, it can now concentrate on 3 spells at once instead of 2.
• The DC of its frightful presence ability Wisdom save increases to 26.
• If a party defeats an Elder Brain variant award them another additional 75,000 XP.

Multiattack. The brainstealer dragon can use its Frightful Presence. It then makes three attacks: two with its claws and one with its tentacles.

Claw. Melee Weapon Attack: +15 to hit, reach 30 ft., one target. Hit: 24 (4d6 + 10) slashing damage. 

Tail. Melee Weapon Attack: +15 to hit, reach 20 ft., one target. Hit: 31 (6d6 + 10) slashing damage. If the target is a creature it must succeed a DC 27 Strength saving throw or be knocked prone.

Tentacles. Melee Weapon Attack: +16 to hit, reach 30 ft., one creature. Hit: 36 (4d12 + 10) psychic damage. If the target is huge or smaller, it is grappled (escape DC 19) and must succeed on a DC 24 Intelligence saving throw or be stunned until this grapple ends.

Extract Brain. Melee Weapon Attack: +16 to hit, reach 15 ft., one incapacitated humanoid grappled by the dragon. Hit: The target takes 110 (20d10) piercing damage. If this damage reduces the target to 0 hit points, the brainstealer dragon kills the target by extracting and devouring its brain.

Frightful Presence. Each creature of the brainstealer dragon's choice that is within 200 feet of the brainstealer dragon and aware of it must succeed on a DC 24 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each turn, ending its effect on itself on a success. If a creature's saving throw is successful or the effect ends, the creature is immune to the brainstealer dragon's Frightful Presence for the next 24 hours.

Mind Blast (Recharge 5-6). The brainstealer dragon magically emits psychic energy in a 200-foot sphere centered on itself. Each creature in that area must succeed on a DC 27 Intelligence saving throw or take 178 (16d10 + 10) psychic damage and be stunned 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.

Enter the description for how bonus actions work for your monster here.

Reaction Name. Enter the description for your action.

The brainstealer dragon can take 5 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 brainstealer dragon regains spent legendary actions at the start of its turn.

Cast a Spell (Costs 2 actions). The brainstealer dragon casts one of their spells.

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

Detect Thoughts. The brainstealer dragon casts detect thoughts.

Tentacle or Tail Attack. The brainstealer dragon makes an attack with its tentacles or its tail.

Psychic Shriek (Costs 2 Actions). The brainstealer dragon emits a wave of psionic energy. Each creature with an Intelligence score of 3 or higher within 30 feet of the brainstealer dragon must make a DC 27 Intelligence saving throw, taking 55 (10d8 + 10) psychic damage on a failed save, or half as much on a successful one. The dragon then teleports up to 120 feet away.

Pyconic Call (Costs 3 Actions). The brainstealer dragon calls out to it Mind Flayer allies, it uses its psionic energy to transport them to assist it in battle (typically 1d6+1 Mind Flayers).

After the brainstealer dragon's mythic trait is active, it can use the following mythic abilities as legendary actions for up to 3 hours.

Mass Dominate Monster (Costs 3 actions). The brainstealer dragon effects up to 10 (or its intelligence modifier) within 60ft of it as if by the dominate monster spell; however the save DC for this attack is 30 instead of its normal DC of 27 and the duration is 48 hours instead of only 1, additionally, the brainstealer can take precise control of the targeted creatures as a bonus action instead of an action, as well it does not use its reaction to make the target use a reaction.