Ancient Tarrasque

Ancient mind. The ancient tarrasque is immune to any effect that affects it's mind.

Colossal. The ancient tarrasque is immune to gaze attacks.

Earth-Shaking Movement. Once per turn as a free action after moving on the ground, the ancient tarrasque can send a shock wave through the ground in a 120-foot-radius circle centered on itself. That area becomes difficult terrain for 1 minute. Each creature on the ground that is concentrating must succeed on a DC 20 Constitution saving throw or the creature's concentration is broken . The shock wave deals 100 thunder damage to all structures in contact with the ground in the area. If a creature is near a structure that collapses, the creature might be buried; a creature within half the distance of the structure's height must make a DC 25 Dexterity saving throw. On a failed save, the creature takes 17 (5d6) bludgeoning damage, is knocked prone, and is trapped in the rubble. A trapped creature is restrained, requiring a successful DC 20 Strength (Athletics) check as an action to escape. Another creature within 5 feet of the buried creature can use its action to clear rubble and grant advantage on the check. If three creatures use their actions in this way, the check is an automatic success. On a successful save, the creature takes half as much damage and doesn't fall prone or become trapped.

Eternal body. When the ancient tarrasque drops to 0 hit points it falls asleep instead of dying and must rest 50 years until it can be awaked again.

God Killer. The ancient tarrasque ignores any immortal or deity resistances or immunities with it actions. It senses if any deity is on the same plane as it is.

Greater Reflective Carapace. Any time the ancient 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 ancient tarrasqueis unaffected. On a 6, the ancient tarrasque is unaffected, and the effect is reflected back at the caster as though it originated from the ancient tarrasque, turning the caster into the target. And any spell that is 6th level or lower, that targets only the ancient tarrasque (not in an area of effect) roll, roll a d8. On a 1 to 7, the ancient tarrasque is unaffected. On a 8, the ancient tarrasque is unaffected, and the effect is reflected back at the caster as though it originated from the ancient tarrasque, turning the caster into the target.

Hope Losing Sight. The first a creature see that isn't legendary must succeed on a DC 22 Wisdom saving throw or become paralyzed from fear for 10 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful, the creature is immune to the Ancient Tarrasque's Hope Losing Sight for the next 12 hours.  but if a creature is paralyzed for the full 10 minutes it gains a long term maddess effect.

Immutable Form. The Ancient Tarrasque is immune to any spell or effect that would alter its form.

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

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

planar bond. The Ancient Tarrasque can't go to another plane of existence.

Tough Hide. The ancient tarrasque ignores any damage less than 20 dealed to it.

Earthbind. Any  flying creature within 400 feet of the ancient tarrasque has it's fly speed reduced to 5 and maximum altitude reduced to 10 feet (putting it within the ancient tarrasque’s reach). Any creature above this altitude at the end of it's turn falls to an altitude of 60 feet automatically.

Greater Siege Monster. The Ancient Tarrasque deals triple damage to objects and structures.

Multiattack. The ancient tarrasque can use its Hope Losing Roar. 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.

Bite. Melee Weapon Attack: +28 to hit, reach 10 ft., one huge target or three targets that are large or smaller within 5 feet of eachother. Hit: 51 (6d12 + 15) 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: +28 to hit, reach 25ft., two targets within 5 feet of eachother. Hit: 53 (8d10 + 15) slashing damage.

Horns. Melee Weapon Attack: +28 to hit, reach 10ft., two targets within 5 feet of eachother. Hit: 61 (8d12 + 15) piercing damage. 

Tail. Melee Weapon Attack: +19 to hit, reach 20ft., one target. Hit: 29 (4d6 + 15) bludgeoning damage. If the target is a creature, it must succeed on a DC 20 Strength saving throw or be lauched 30 feet away from the Ancient Tarrasque and knocked prone.

Hope Losing Roar. Each creature that is within 1000 feet of the Ancient Tarrasque and aware of it must succeed on a DC 22 Wisdom saving throw or become paralyzed from fear for 5 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful, the creature is immune to the Ancient Tarrasque's Hope Losing Sight for the next 12 hours. 

Swallow. The Ancient Tarrasque makes one bite attack against each huge or smaller creature that 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 154 (28d10) acid damage at the start of each of the tarrasque's turns, this ignores acid damage resistance.If a magic item stays swallowed for the time in the table below it will loose it's magic.

• Common takes 2 turn.
• uncommon takes 8 turns.
• rare takes 5 minutes.
• very rare takes 1 hour.
• legendery takes 12 hours.
• Artifacts takes 5 days.

The ancient tarrasque's stomach is immune to magic and can't be teleported in or out, If the ancient tarrasque takes 600 damage or more on a single turn from a creature inside it, the ancient 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 ancient tarrasque. If the ancient 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 ancient tarrasque can take 10 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 ancient tarrasque makes one claw attack or tail attack.

Move. The ancient tarrasque moves up to half its speed.

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

Overwhelming Gaze (Costs 3 Actions). The ancient tarrasque fixes its gaze on one creature it can see within 70 feet of it. The target must succeed on a DC 30 Wisdom saving throw against this or fall unconscious from fear for 1 minute. The unconscious target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a target's saving throw is successful or the effect ends for it, the target is immune to the ancient tarrasque's gaze for the next 12 hours and become frightened for 1 turn.