Awakened Tarrasque

Terrible Transformation (Mythic Trait; Recharges after a Short or Long Rest). When the tarrasque is reduced below half it's hit points, the tarrasque will cast Tenser's Transformation as a free action gaining all the benefits. This spell does not count toward's its concentration, and the tarrasque will maintain the benefit of the spell and it's mythic actions unless dispelled by a DC 22 dispel or is suppressed by an anti-magic field. 

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

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

Reflective Carapace. Any time the tarrasque 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 tarrasque is unaffected. On a 6, the tarrasque is unaffected, and the effect is reflected back at the caster as though it originated from the tarrasque, turning the caster into the target.

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

Trample. When the tarrasque enters any enemy's space for the first time on a turn, the enemy must succeed on a DC 19 Dexterity saving throw or take 31 (6d6+10) bludgeoning damage and be knocked prone

Spellcasting. The tarrasque is an 18th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 17, +9 to hit with spell attacks). The tarrasque can cast disguise self and invisibility at will and has the following wizard spells prepared:

Cantrips (at will): fire bolt, light, mage hand, prestidigitation, shocking grasp

1st level (4 slots): detect magic, identify, absorb elements, magic missile

2nd level (3 slots): detect thoughts, mirror image, misty step

3rd level (3 slots): counterspell, fly, nondetection*

4th level (3 slots): banishment, fire shield, polymorph

5th level (3 slots): planar binding, scrying, wall of force

6th level (1 slot): tenser's transformation

7th level (1 slot): teleport

8th level (1 slot): mind blank*

9th level (1 slot): time stop

*The tarrasque casts these spells on itself before combat.

Multiattack. The tarrasque can use its Frightful Presence. It then makes five attacks: one with its bite, two with its claws or rocks, one with its horns, and one with its tail. It can use its Swallow 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.

Rock Throw: The Tarrasque picks up a large rock and throws it. +19 to hit. Range 400/1000 ft., one target. Hit: 38 (4d12 +10) bludgeoning damage.

Frightful Presence. Each creature of the tarrasque'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 tarrasque 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 tarrasque's Frightful Presence for the next 24 hours.

Swallow. The tarrasque 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 tarrasque, and it takes 56 (16d6) acid damage at the start of each of the tarrasque'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.

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

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

Move. The tarrasque moves up to half its speed.

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

The tarrasque can take 3 mythic actions, choosing from the options below. Only one mythic action option can be used at a time and only at the end of another creature's turn. The tarrasque regains spent mythic actions at the start of its turn.

Tail Sweep (Recharge 5-6). The tarrasque makes one tail attack against all valid targets currently within its reach up to 8. It makes one attack roll and one damage roll, and uses the result for each attack.

Cast a spell.(1-3 actions) The tarrasque can cast a spell of 6th level or lower. Costing half the level rounded up in action points. 

Energy Breath (Recharge 6). The tarrasque exhales pure energy in a 120-foot cone. Each creature in that area must make a DC 24 Constitution saving throw, taking 91 (26d6) radiant damage on a failed save, or half as much damage on a successful one.