Actions

Multiattack.

The tarrasque makes five attacks: 3 light attacks, 1 heavy attacks, and 1 special attack.  

Melee.

Claw, Light. Melee Weapon Attack: +10 to hit, reach 15 ft, one target. Hit: 24 4d6+10 slashing damage.
Tail, Heavy. Melee Weapon Attack: +10 to hit, reach 20 ft, one target. Hit: 24 4d8+10 bludgeoning damage.
If the target is a creature, it must succeed on a DC 20 Strength saving throw or be knocked prone.
Bite, Heavy. Melee Weapon Attack: +10 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.
Swallow, Special. The tarrasque makes one bite attack against a Large or smaller creature it is grappling. On a hit, 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 30 5d10 acid + 30 5d10 force 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, it must succeed on a DC 20 Con save at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the tarrasque.
Horrifying Scream (recharge 5-6). Each creature that can hear the tarrasque must succeed on a DC 13 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns ending the effect on itself on a success.  

Ranged.

Rail Gun, Light. Ranged Weapon Attack: +10 to hit, range 100/400 ft, one target. Hit: 17 2d6+5 piercing + 17 2d6+5 lightning damage.
Mortar Strike, Heavy. The tarrasque's uses its spinal cannons to rain fire in a 20-foot-wide line, starting 45 feet away and continuing for 90 feet.
Each creature in that line must make a DC 18 Dexterity saving throw. On a fail, they take 14 2d8+5 fire + 14 2d8+5 force damage or half as much on a success.
Gravity pull, Special. The tarrasque opens its chest, revealing a gravity vortex. Each creature within a 120-foot-cone of the tarrasque must make a DC 20 Dexterity saving throw. On a fail, they take 21 6d6 bludgeoning damage and are pulled 20 feet forward (landing prone on a fail of 12 or less).

Legendary Actions

The tarrasque can take 3 legendary actions, choosing from the options below.
Only one legendary action 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 light attack.
Move. The tarrasque moves up to half its speed.
Chomp (Costs 2 Actions). The tarrasque makes one bite attack or uses its Swallow.
Salvo (Costs 2 Actions). The tarrasque makes one mortar attack.