Actions

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.   Bite. Melee Weapon Attack: +16 to hit, reach 5 ft, one target. Hit: 36 4d12+10 piercing damage.
If the target is a creature, it is grappled (escape DC 19). Until this grapple ends, the target is restrained, and the tarrasque can't bite another target.
Claw. Melee Weapon Attack: +16 to hit, reach 10 ft, one target. Hit: 28 4d8+10 slashing damage.
Horns. Melee Weapon Attack: +16 to hit, reach 5 ft, one target. Hit: 32 4d10+10 piercing damage.
Tail. Melee Weapon Attack: +16 to hit, reach 15 ft, one target. Hit: 24 4d6+10 bludgeoning damage.
If the target is a creature, it must succeed on a DC 19 Strength saving throw or be knocked prone.   Frightful Presence. Each creature of the tarrasque's choice within 80 feet of it and aware of it must succeed on a DC 14 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.

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 claw attack or tail attack.
Move. The tarrasque moves up to half its speed.
Chomp (Costs 2 Actions). The tarrasque makes one bite attack.