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

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

Siege Monster. The fisher deals double damage to objects and structures.

Multiattack. The fisher can use its Frightful Presence. It then makes three attacks with its bite. It can also use its Swallow instead of its bite.

Bite. Melee Weapon Attack: +19 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 fisher can't bite another target.

Frightful Presence. Each creature of the fisher's choice within 120 feet of it and aware of it must succeed on a DC 18 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 fisher 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 fisher's Frightful Presence for the next 24 hours.

Swallow. The fisher 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 fisher takes 60 damage or more on a single turn from a creature inside it, the fisher 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 fisher. If the fisher 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 fisher 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 fisher regains spent legendary actions at the start of its turn.

Attack. The fisher makes one bite attack.

Move. The fisher moves up to half its speed.

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