Beyonder

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

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

Reflective Carapace. Any time the Beyonder is targeted by a magic missile spell, a line spell, or a spell that requires a ranged attack roll, roll a d6. On a 1 to 5, the Beyonder is unaffected. On a 6, the tarrasque is unaffected, and the effect is reflected back at the caster as though it originated from the Beyonder, turning the caster into the target.

Siege Monster. The Beyonder deals double damage to objects and structures.

Innate Spellcasting. Because of the Beyonder's innate spellcasting It can innately cast spells, requiring no material components. 

Spellcasting. The Beyonder is a 20th-level spellcaster. It's spellcasting ability is Charisma (spell save DC 24, +16 to hit with spell attacks). The Beyonder has the following spells prepared.

1st level spells (at will) Bane, Command, Charm Person, Detect Magic, Sleep, Ray of Sickness

2ed level spells (at will) Hold Person, Silence, Melf's Acid Arrow, Blindness/Deafness, Crown of Madness, Enthrall, Suggestion, Scorching Ray

3rd level spells (at will) Bestow Curse, Dispel Magic, Counterspell, Slow, Fireball Lightning Bolt

4th level spells (8 slots) Compulsion, Confusion, Locate Creature, Blight, Wall of Fire, Conjure Minor Elemental, Summon Greater Demon, 

5th level spells (8 slots) Contagion, Cloudkill, Scrying, Conjure Elemental, Insect Plague, Dominate Person, Cone of Cold, Hold Monster, Mislead, Wall of force, Wall of Stone

6th level spells (6 slots) Harm, Disintegrate, Eyebite, Wind Wall, Chain Lightning, Mass Suggestion, 

7th level spells (6 slots) Finger of Death, Fire Storm, Plane Shift, Crown of Stars, Power Word Pain

8th level spells (5 slots) Dominate Monster, Power Word Stun, Maddening Darkness, Feeblemind, Glibness, Control Weather, Earthquake, Tsunami, Mind Blank, Mass Summon Greater Demons, Mass Summon Elemental

9th level Spells (4 slots, but can only use each spell once per day) Power Word Kill, Psychic Scream, Weird, Wish, Meteor Storm, Summon Elder Elemental

The Great Old One. The Great Old one's blood pumps threw the veins of this creature. Any creature of a challenge rating lower than the Beyonder summoned or conjured by the Beyonder is under complete control of the Beyonder, no concentration needed.

Multiattack. The Beyonder can use its Frightful Presence. It then makes five attacks: one with its bite, two with its claws, one with its horns, and one with its tail. It can use its Swallow, Tentacles, or Extract Brain instead of its bite.

Bite. Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 36 (4d12 + 10) piercing damage. If the target is a creature, it is grappled (escape DC 20). Until this grapple ends, the target is restrained, and the tarrasque can't bite another target.

Claw. Melee Weapon Attack: +19 to hit, reach 15ft., one target. Hit: 28 (4d8 + 10) slashing damage.

Horns. Melee Weapon Attack: +19 to hit, reach 10ft., one target. Hit: 32 (4d10 + 10) piercing damage.

Tail. Melee Weapon Attack: +19 to hit, reach 20ft., one target. Hit: 24 (4d6 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 20 Strength saving throw or be knocked prone.

Frightful Presence. Each creature of the Beyonder's choice within 120 feet of it and aware of it must succeed on a DC 17 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, with disadvantage if the Beyonder is within line of sight, 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 Beyonder's Frightful Presence for the next 24 hours.

Swallow. The Beyonder makes one bite attack against a Large or smaller creature it is grappling. If the attack hits, the target takes the bite's damage, the target is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the Beyonder, and it takes 56 (16d6) psychic damage at the start of each of the Beyonder's turns.

If the tarrasque takes 60 damage or more on a single turn from a creature inside it, the tarrasque must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the tarrasque. If the tarrasque dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.

Tentacles. Melee Weapon Attack: +19 to hit, reach 20 ft., one creature. hit: 36 (4d12+10) psychic damage. If the target is huge or smaller, it is grappled (escape DC 20) and must succeed on a DC 18 Intelligence saving throw or be stunned until the grapple end.

Extract Brain. Melee Weapon Attack: +19 to hit, reach 5 ft., one creature grappled and stunned by the Beyonder's tentacles attack, on a hit the creature takes (10d10) piercing damage. If this damage reduces the target to 0 hit points, the Beyonder kills the target, extracting and devouring it's brain.

Mind Blast (Recharge 5-6). The Beyonder magically emits psychic energy from its body. Each creature within 300 feet for the Beyonder must make a DC 17 Intelligence saving throw or take 8d12+10 psychic damage and be stunned for 1 minute. A target can repeat the saving throw at the end of each of it's turns, ending the effect on itself on a success.

The Beyonder 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 Beyonder regains spent legendary actions at the start of its turn.

Attack. The Beyonder makes one claw attack or tail attack.

Move. The Beyonder moves up to half its speed.

Chomp (Costs 2 Actions). The Beyonder makes one bite attack or uses its Swallow.

Tentacles (Costs 2 Actions). The Beyonder makes one Tentacles attack or uses its Extract Brain.

Cast a spell (Costs 1-5 actions). The Beyonder expends a spell slot to cast a 1st-5th level spell, doing so costs 1 legendary action per level of the spell.