Assimilated Tarrasque

Warforged Resilience. The warforged has advantage on saving throws against being poisoned and is immune to disease. Magic can't put it to sleep.

Regeneration. The warforged regains 50 hit points at the start of its turn if it has at least 1 hit point.

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.

Multiattack. The tarrasque 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 instead of its bite, and it can use its Line Blast instead of its claws.

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 15 ft., one target. Hit: 28 (4d8 + 10) slashing damage.

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

Tail. Melee Weapon Attack: +19 to hit, reach 20 ft., 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 tarrasque's choice within 120 feet of it and aware of it must succeed on a DC 27 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.

Line Blast. The Warforged holds its arms up, and a 60 ft long by 5 feet wide line of a random substance shoots out of openings on its hands in a direction of its choice (roll a d4). 

• Fire: All creatures within range must roll a DC 27 Dexterity saving throw, taking 6d10 fire damage on a failed save or half as much on a successful save.
• Ice: All creatures within range must roll a DC 27 Constitution saving throw, taking 6d8 cold damage on a failed save or half as much on a successful save. All creatures that fail the save also have disadvantage on their next attack roll.
• Acid: All creatures within range must roll a DC 27 Dexterity saving throw, taking 6d6 damage on a failed save or half as much on a successful save. All creatures that fail the save also take 2d6 acid damage each turn for the next two turns.
• Lightning: All creatures within range must roll a DC 27 Dexterity saving throw, taking 6d8 damage on a failed save or half as much on a successful save. All creatures that fail the save cannot take reactions until the start of their next turn.

Protection. When an attacker the warforged can see makes an attack roll against a creature within 5 feet of the warforged, the warforged can impose disadvantage on the attack roll.

The Tarrasque can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time. The Drone regains spent legendary actions at the start of its turn.

• Absorption. Upon taking damage of any type, the Tarrasque may use a legendary reaction to roll a d6. On a 4 or higher, the Tarrasque gains immunity to that damage type for the next minute. Otherwise, it gains resistance to that damage type for the next minute. For bludgeoning, slashing, or piercing damage from magical weapons, this reaction can only give resistance.
• Battle Reincarnation (Costs 2 Actions). Upon encountering a creature that is unconscious, or forcing a creature to become unconscious through the use of spells or falling to 0 HP, the Drone may use its reaction or an action to stabilize it if the Tarrasque is within 5 feet of the creature by imbuing some of its innate power into it. The unconscious creature automatically becomes stable and immediately rolls a DC 15 Constitution saving throw. On a failure, the creature enters a form of stasis that can only be broken through spells that would remove a curse, remaining unconscious during this time. The creature continues to roll this save at the start of each of its turns, with the DC increasing by 1 each round.
  
  Upon failing the saving throw, the creature gains the benefits of a long rest, and cannot wake up for 24 hours or until the stasis is broken. Once 24 hours have passed, the stasis ends, the creature becomes conscious and rises as a version of a warforged soldier equal in level or CR rating to the creature it was originally. Some creatures are incompatible with the effects granted by the warforged. Any nonhumanoid creature or humanoid creature for which there is no replacement stat for the warforged gains two forms of indefinite madness: "The character suffers from partial amnesia. The character knows who he or she is and retains racial traits and class features, but doesn't recognize other people or remember anything that happened before the madness took effect" and "Achieving my goal is the only thing of interest to me, and I'll ignore everything else to pursue it". Any nonhumanoid creature with an intelligence of 6 or lower is immune to the madness effect but instead is forced under the control of the creature that reincarnated it, and any creature targeted by this ability that is undead dies instantly and cannot be resurrected short of a wish spell.