Astral Tarrasque

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

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

Reflective Carapace. Any time the Astral tarrasque is targeted by a magic missile spell, a line spell, or a spell that requires a ranged attack roll from the outside, roll a d6. On a 2 to 5, the Astral tarrasque is unaffected. On a 6, the Astral tarrasque is unaffected, and the effect is reflected back at the caster as though it originated from the  Astral tarrasque, turning the caster into the target. If the spell does damage, 1d6 fire damage is added, and 1d6 for each damage type the Tarrasque is resistant to thanks to its adaptive plating with each d6 doing damage of the triggering resistance.

Adaptive Plating. When the Astral Tarrasque takes damage from any source other than psychic. It takes the damage, then gains resistance to that damage type until the start of its next turn. 

Astral Hide. When the Astral Tarrasque is dealt damage, it negates some of the damage's potency. Any incoming bludgeoning, piercing, slashing, cold, acid, lightning, fire or thunder damage is reduced by 25, before any resistances apply. If the Astral Tarrasque would take 100 damage at once before triggering this effect the astral hide instead absorbs one half of the damage, and ceases to function until the end of the Astral Tarrasque's next turn. For each part that is currently broken of the segmented form the damage negation reduces by 5. 
( Clarification Examples: If  an attack did 18 slashing damage and 13 cold damage. 25 damage would be negated leaving either 6 slashing, or 6 cold to hit the  Astral Tarrasque.  If a paladin's smite did 11 bludgeoning damage, and 25 radiant. All of the bludgeoning damage would be negated, but all 25 radiant damage would pass through as it is not one of the types Astral Hide affects.)

Final Form. When the Astral Tarrasque drops below 100 hp. It enters a Celestial frenzy. Its size reduces to medium, and it glows with radiant energy. While in this form, the Astral Tarrasque's AC increases to 30, it gains proficiency in all saving throws (Prof bonus of +10) and advantage on all ability checks and attack rolls. The Astral Tarrasque gains resistance to all damage it did not already have until it dies or leaves this form.These resistances are treated as if provided by adaptive plating for the purposes of the Astral Tarrasque's actions. Upon entering this form, the Astral Tarrasque instantly regains all of it Recharge abilities, and takes an additional turn. When a creature starts its turn within 30ft of the Astral Tarrasque in this form or enters the range for the first time on its turn, it must make a DC 20 Constitution saving throw or take 5d12 radiant damage and become blinded until the start of its next turn. Any creature swallowed by the  Astral Tarrasque when it transforms takes 10d6 Force damage and is ejected to a space adjacent to the Astral Tarrasque.

Inevitable. The Astral Tarrasque has ways of dealing with minor set backs. If an effect would instantly drop the Astral Tarrasque to 0 hit points it instead drops to 1 and triggers its Final form. If an effect would cause the Astral Tarrasque to change forms, it may use an action on its turn to revert. If a magic obstacle such as a wall of force, force cage, or resilient sphere prevents the Astral Tarrasque from reaching its target the it may use its horn attack or both of its claw attacks to destroy the obstacle. If an effect would reduce the Astral Tarrasques movement speed to drop 0 it is instead reduced to half. If the creature would be send to another plane of existence, it does not become incapacitated, and can leave the plane using a charge of its Spatial Rend ability. 

Segmented. The Astral Tarrasque has certain abilities which rely on certain parts of the body as denoted by a (Part Name) If that part is broken the Astral Tarrasque can't use the ability. If a creature declares a particular target on the Astral Tarrasque's body, it can attempt to destroy a one of these parts. These parts are unaffected by Aoe spells, but do not benefit from the Astral Tarrasque's reflective carapace, or its Astral hide. A part is considered broken if its hp hits 0. Any effect that would cause decapitation or limb loss (such as sword of sharpness) instead instantly breaks one of the parts below.If the Astral Tarrasque starts its turn with all of its parts broken roll 1d4. The corresponding part below is regrown at full health.

1.Horns: Ac 27. Hp 200.  Vulnerable to Thunder Damage.
2.Left Claw: Ac 27. Hp 100. Immune to Lightning Damage, vulnerable to cold Damage
3.Right Claw: Ac 27. Hp 100. Immune to Necrotic Damage, vulnerable to radiant Damage
4.Tail: AC 20. Hp 300. Vulnerable to Slashing Damage.

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

Astral Armada. The Astral Tarrasque can adapt to fight numerous foes. When faced against an overwhelming number enemies the Astral tarrasque can take additional turns in initiative. To determine the appropriate number of turns to give the Astral Tarrasque, add together the total Character level of players and CR of creatures it will be facing and divide the total by 50 round down, (Minimum 1). The resulting number will be the appropriate number of turns for the Astral Tarrasque to take per round of initiative. Roll initiative for each turn separately then drop the lower initiatives until no two Astral Tarrasque turns are adjacent being careful to avoid having one at both top and bottom of initiative. Reroll the lowest initiative if necessary to avoid such a configuration. If the configuration is impossible remove turns until it is possible.

Multiattack. The Astral Tarrasque can use its Frightful Presence. It can use one of its recharge abilities if it has it. It then makes five attacks: one with its bite, one with its left claw, one with its right claw, one with its horns, and one with its tail. It can use its Swallow instead of its bite. It can replace two attacks with a usage of Empyrean Fury.

Bite. Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 36 (4d12 + 10) piercing damage+ 22(4d10) poison 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. This attack does extra damage equal to 1d6 for each damage type the Astral Tarrasque resists due to its adaptive plating.

Left Claw( Left Claw). Melee Weapon Attack: +19 to hit, reach 15ft., one target. Hit: 28 (4d8 + 10) slashing damage and 28 (8d6) lightning damage. The target must then make a DC 20 Constitution Saving throw or become paralyzed until the end of the Astral Tarrasque's next turn. On a critical hit, the target automatically fails this saving throw.

Right Claw( Right Claw). Melee Weapon Attack: +19 to hit, reach 15ft., one target. Hit: 28 (4d8 + 10) slashing damage and 18(4d8) necrotic damage. The Astral Tarrasque then regains hitpoints equal to the amount of necrotic damage dealt. On a critical hit, the Astral Tarrasque regains hitpoints equal to the total damage dealt not just the necrotic damage.

Horns( Horns). Melee Weapon Attack: +19 to hit, reach 10ft., one target. Hit: 32 (4d10 + 10) piercing damage and 22(4d10) force damage. On a critical hit the target becomes impaled upon the horns, dealing an additional 22 (4d10) piercing damage. At the start of each of that creatures turns it must make a DC 20 constitution constitution saving throw or take the additional damage again ending the effect on a success.

Tail. (Tail) Melee Weapon Attack: +19 to hit, reach 20ft., one target. Hit: 24 (4d6 + 10) piercing damage. The Tail can hit creatures in the Ethereal Plane. On a hit, If the creature is in the ethereal plane the tail can pull them into to Astral Tarrasques current plane of existence, or if the creature is in the same plane of existence as the Astral Tarrasque it can push them into the Ethereal Plane. In either case, the target must make a DC 20 Charisma Save or be forced into the respective plane. Being forced into the Ethereal plane is a more consumptive experience than being pulled out, a creature who fails their Charisma Save and is pushed into the ethereal plane takes a level of exhaustion. Such a creature returns to the Plane of existence they were pushed from at the start of their next turn.

Frightful Presence. Each creature of the Astral Tarrasque's choice within 120 feet of it and aware of it must succeed on a DC 20 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 Astral 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 Astral Tarrasque's Frightful Presence for the next 24 hours.

Swallow. The Astral 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 Astral Tarrasque, and it takes 26(4d12) acid damage and 26(4d12) cold damage  at the start of each of the Astral 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.

Empyrean Fury.  The Astral Tarrasque expels 10 bolts of cosmic energy.  Each bolt hits a creature of choice that the Astral Tarrasque can see within 300ft. A bolt deals 1d12 + 5 force damage to its target. The darts all strike simultaneously, and you can direct them to hit one creature or several. These bolts can be stopped by any means that would normally stop the magic missile spell. A creature hit by at least one bolt must make a Intelligence Save with DC equal to half the damage taken. On a failure the creature is pulled up to 50 ft in a direction of the Astral Tarrasques choice.

Recharge Abilities. (Roll 1d6, and recharge one ability according to the number rolled. The Astral Tarrasque can use at most one of its recharge abilities per turn regardless of how many are recharged.)

Astral Annihilation(Recharge 6). The  Astral Tarrasque produces a stream of concentrated astral distortion in a line 500ft long and 10 ft wide. Each creature within the line must succeed on a DC 25 Dexterity saving throw or take 100 force damage and become trapped in a gravity well. A creature who succeeds on its save takes half as much damage and does not become trapped. While in a gravity well the character is considered restrained and under the effects of the slow spell. On their turn they may use their action to attempt to escape, by making a DC 20 Athletics check. When the creature escapes the gravity well dissipates.

Spatial Rend(Recharge 5)(Left Claw or Right Claw). The Astral Tarrasque, rips a hole through reality in front of it. Until the end of its next turn the Tarrasque can create another hole in a spot it could see when the first hole was made within 1 mile. Each hole lasts until the end of the Tarrasques next turn. A creature can use 10ft of movement to move from one hole to the other. Any creature other than the Astral Tarrasque, that begins or ends its turn within 20 ft of one of these holes must make a DC 20 intelligence saving throw, taking 6d10 psychic damage on a failed save or half as much on a successful save.  A creature which is inside these holes when they close must make a DC 20 dexterity saving throw or take 4d12 force damage and be sucked into a random location on the Astral Plane. On a successful save the creature instead takes no damage, and emerges from one of the portals of their choice before they close.

Cosmic Obliteration(Recharge 4) (Horns). The Astral Tarrasque releases a blast of Cosmic energy in a 300ft cone. Each target within range is overwhelmed with cosmic energy and must make a DC 20 Constitution Saving throw or take 20d6 Radiant damage and become blind until the end of their next turn. On a successful save a creature takes half as much damage and is not blinded.

Spectral Rejuvenation(Recharge 3). The Astral Tarrasque removes any negative condition effecting it, and reforms up to two of its broken parts out of the surrounding starlight each part regains full health. The starlight parts lose their normal immunities and gain vulnerability to force and necrotic damage.

Siphoning Radiance(Recharge 2). The Astral Tarrasque unleashes a 60 ft radius sphere of bright light which seems to sap the minds of the creatures around it until the end of its turn. Each creature within range other than the Astral Tarrasque must make a DC 20 intelligence saving throw or suffer a level of exhaustion. This Exhaustion lasts one minute. 

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.

Blitz (Costs 2 Actions). The Astral Tarrasque uses its Empyrean Fury Ability.

Charging Pulse (Costs 3 Actions). The Astral Tarrasque unleashes a wave of destruction in a 100 ft radius centered on itself. Every other creature in the area must make a DC 20 Strength saving throw. On a failed save, a creature takes an amount of damage based on how far away it is from the Astral Tarrasque, as shown in the following table and is thrown backwards 30 ft. On a successful save, a creature takes half as much damage and is not thrown backwards.

Distance from Origin — Damage:

Inside the Astral Tarrasque - 32 x the number of turns since combat started

20 ft. away or closer — 16 x the number of turns since combat started

21 to 50 ft. away — 8 x the number of turns since combat started

51 to 80 ft. away — 4 x the number of turns since combat started

81 to 100 ft. away — 2 x the number of turns since combat started.