Arua of the apocalypse. Each creature within 2500 miles of the swarm is automatically  aware of the swarm must succeed on a DC 50 Wisdom saving throw or if they fail by 20 or more they die of fear, any creature that is immune being frightened is immune to this effect.  If a creature's saving throw is successful, the creature is immune to The swarm's arua of the apocalypse for the next 24 hours, but gains a indefinite madness flaw from the Indefinite Madness table.

Legendary Resistance (300/Day). If The swarm fails a saving throw, it can choose to succeed instead.

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

Siege Monster. The swarm deals quadruple damage to objects and structures.

Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a gargantuan swarm of tarrasques. The swarm gain temporary hit points. 

Reflective Carapace. Any time The swarm 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, The swarm is unaffected. On a 2,3,4,5,6, The swarm is unaffected, and the effect is reflected back at the caster as though it originated from The swarm, turning the caster into the target.

colossal size. the swarm has resistance against any form of area attacks.

planar bond. the swarm can't go to another plane of existence.

storm wakker (1/Day). the swarm can cause a storm to appear of 500 miles by 500 miles centered where it stands for 24 hours.

Freedom of Movement. The swarm ignores difficult terrain, and magical effects can't reduce they're speed or cause them to be restrained. they  can spend 5 feet of movement to escape from restraints or being grappled.

Multiattack. The swarm makes five hundred attacks: hundred with its bites, two hundred with there claws, hundred with there horns, and hundred with there tails. they can use there Swallow instead of there bites.

Bites. Melee Weapon Attack: +38 to hit, reach 10 ft., one target. Hit: 2500 (400d12 + 100) piercing damage, or 1250 (200d12 + 100)  piercing damage if the swarm has half of its hit points or fewer, If the target is a creature, it is grappled (escape DC 20). Until this grapple ends, the target is restrained, and the swarm can't bite if they have a hundred targets grappled.

Claws. Melee Weapon Attack: +38 to hit, reach 15ft., one target. Hit: 170 (400d8 + 100) slashing damage, or 85 (200d8 + 10)  slashing damage if the swarm has half of its hit points or fewer. the swarm may launch the target knock back 50 feet than the target must succeed on a DC 20 dexterity saving throw or be knock prone.

Crushing charge. the swarm moves 120 feet straight toward a target and then hits it with a horns attack on the same turn, the target takes an extra 2100 (400d10 + 100) piercing damage, or 105 (200d10 + 100)  piercing damage if the swarm has half of its hit points or fewer. If any creature or object that isn't bigger 50ft/50ft big is in the path the swarm makes makes automatically horns attack agianst it.

Horns. Melee Weapon Attack: +38 to hit, reach 10ft., one target. Hit: 2100 (400d10 + 100) piercing damage, or 105 (200d10 + 100)  piercing damage if the swarm has half of its hit points or fewer.

Tails. Melee Weapon Attack: +38 to hit, reach 20ft., one target. Hit: 1300 (400d6 + 100) bludgeoning damage, or 650 (200d6 + 100)  bludgeoning damage if the swarm has half of its hit points or fewer, If the target is a creature, it must succeed on a DC 40 Strength saving throw or be knocked prone.

Swallow. The swarm 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 swarm takes 60 damage or more on a single turn from a creature inside it, the swarm 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 swarm. when a creature is regurgitate the swarm can make automatically a bites attack against it. If the swarm dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.

Roar of doom. The swarm roars in a direction, each creature in a 60-foot cone, must make a DC 30 strenthg saving throw, taking 43 (10d6) force damage and launched back 200 feet on a failed save, or half as much damage and only 50feet back on a successful one.

The swarm can take 30 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 swarm regains spent legendary actions at the start of its turn.

Attack (Costs 10 Actions). The swarm makes hundred claw attack or tail attack.

Move (Costs 10 Actions). The swarm moves up to half there speed.

Chomp (Costs 20 Actions). The swarm makes hundred bite attack or uses there Swallow.

Lightning (Costs 30 Actions). the swarm cause hunderd target within 100 miles of the swarm must succeed on a DC 21 dexterity saving throw, on a failed save the target gets hit by lightning dealing 10d8 lightning damage, or half as much damage on a successful one.