Awoken Tarrasque

Bloodied Frenzy (Recharges after a Short or Long Rest): If the Tarrasque would be reduced to 0 hit points, its current hit point total instead resets to 575 hit points and it regains any expended uses of Legendary Resistance. Additionally, the Tarrasque can now use the options in the "Mythic Actions" section for 1 hour. Award a party an additional 155,000 XP (310,000 XP total) for defeating the Tarrasque after its Bloodied Frenzy activates.

Colossal Size: The Tarrasque occupies a 25x25 grid of space and ignores any difficult Terrain smaller then 15ft. across. A Huge or smaller creature can move through the Tarrasque's space without movement penalty and vice versa. If it has grappled a Huge or smaller creature it can still move its full speed. 
  Ranged attacks do not have disadvantage on the Tarrasque when a creature is within 5ft of it, attacking from long range or if the Tarrasque is prone. 
  The Tarrasque has advantage on Strength based ability checks and Strength saving throws.

Elder of Annihilation:  Damage from the Tarrasque's natural weapons ignores all resistances and immunities and cannot be reduced or absorbed in any way.

Godsbane: The Tarrasque deals maximum damage to aspects, gods, titans, celestials and fiends. If the Tarrasque reduces an aspect, god, titan, celestial, fiend or undead creature to 0 hit points, it kills them instantly, bypassing any feature that would prevent this.

Immortal Nature: The Tarrasque requires no air, sustenance or sleep.

Immutable Fixture: The Tarrasque is immune to any spell or effect that would alter its form or unwillingly send it to any other plane of existence. 

Legendary Resistance (5/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.

Regeneration. The Tarrasque is immune to all diseases and regains 30 hit points at the start of it's turn. If the Tarrasque loses a limb or body part, it can reattach the severed member instantly by holding it to the stump. Otherwise the lost portion regrows in 1d6 minutes (the detached piece dies and decays as normal). If the Tarrasque takes acid damage, it only regains half the points given by this trait at the start of  its next turn.
  No magic, spell or spell-like effect can prevent this trait or reduce the Tarrasques hit point total. If subjected to an effect that would kill the Tarrasque instantly it instead drops to 0 hit points, unconscious for 1d6+ 5 rounds or until or starts it’s turn with 50 or more hit points. The Tarrasque dies only if a Wish spell or a successful Divine Intervention from a character with at least 17 levels in Cleric is used when it has 0 hit points. 

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

Multiattack. The Tarrasque can use its Frightful Presence. It then makes five melee attacks: one with its bite, two with its claws, one with its horns, and one with its tail. It can use its Swallow instead of its bite. Alternatively it makes two ranged attacks with debris throw. 

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 27). 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 27 Strength saving throw or be knocked prone. 

Debris Throw. Ranged Weapon Attack: +14 to hit, range 240/1200 ft., one or two adjacent targets. Hit: 56 (7d12 + 10) bludgeoning damage. If hit the target must succeed on a DC 27 Strength saving throw or be knocked prone.

Frightful Presence. Each creature of the Tarrasque’s choice within 300 feet of it and aware of it must succeed on a DC 18 Wisdom saving throw or become frightened. If this saving throw result is 13 or lower the creature is also paralyzed for the duration of the effect. 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 the saving throw result 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 Huge 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.  A creature left with 0 hp after taking this acid damage disintegrates and cannot be brought back by any means. Any objects not worn or carried that take this acid damage are destroyed, including artifacts.
  The Tarrasques Gullet can hold 2 Huge, 8 Large, 32 Medium, 128 Small, or 512 Tiny or smaller creatures. 

Underfoot Stomp. Melee Weapon Attack: +19 to hit, reach 0ft., one or two adjacent targets. Hit: 42 (5d12+10) bludgeoning damage. If hit a creature must succeed on a DC 27 Strength saving throw or it is knocked prone and is incapacitated until the end of its next turn. 

Deadly Reflexes. In response to an enemy it can see moving within 15 ft of it, the Tarrasque can make an opportunity attack with a claw or tail. 

The Tarrasque can take 4 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 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.

Debris Throw (Costs 2 Actions). The Tarrasque makes one debris throw attack.

Earthbind Aura  (Costs 2 actions). Each creature of the Tarrasque’s choice within 1500 feet of it must succeed on a DC 28 Strength saving throw or its flying speed (if any) is reduced to 0 feet for the next minute. An airborne creature affected by this spell safely descends at 60 feet per round until it reaches the ground or the minute ends. Airborne objects automatically fail the saving throw and plummet to the ground. 

Frenzied attacks  (Costs 3 actions). The Tarrasque makes a claw attack against each creature within 15 feet of itself.

Rampage (Costs 3 actions). The Tarrasque moves up to it's speed in a straight line, ignoring difficult terrain. It can move through the space of a Huge or smaller creature unimpeded and end the movent there if it wishes. A creature whose space the Tarrasque enters for the first time on this turn must make a DC 21 Dexterity saving throw or take 35 (6d10) bludgeoning damage and be knocked prone.