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

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

Reflective Carapace. Any time the zombie 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 tarrasque is unaffected. On a 6, the tarrasque is unaffected, and the effect is reflected back at the caster as though it originated from the tarrasque, turning the caster into the target.

Siege Monster. The  zombie tarrasque deals double damage to objects and structures.

Disgorge Zombie. As a bonus action, the zombie tarrasque can disgorge a normal ogre zombie, which appears in an unoccupied space within 10 feet of it. The disgorged ogre zombie acts on its own initiative count. After an ogre zombie is disgorged, roll a d6. On a roll of 1, the zombie tarrasque runs out of zombies to disgorge and loses this trait. If the zombie tarrasque still has this trait when it dies, 1d4 normal ogre zombies erupt from its corpse at the start of its next turn. These ogre zombies act on their own initiative count.

Undead Fortitude. If damage reduces the zombie tarrasque to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the zombie drops to 1 hit point instead.

Multiattack. The zombie 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.  The zombie tarrasque can replace two melee attacks with a single ranged Hurl Earth attack. 

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 tarrasque can't bite another target.

Claw. Melee Weapon Attack: +19 to hit, reach 15ft., one target. Hit: 28 (4d8 + 10) slashing damage.

Horns. Melee Weapon Attack: +19 to hit, reach 10ft., one target. Hit: 32 (4d10 + 10) piercing damage.

Tail. Melee Weapon Attack: +19 to hit, reach 20ft., one target. Hit: 24 (4d6 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 20 Strength saving throw or be knocked prone.

Hurl Earth. Ranged Weapon Attack: +19 to hit, reach 60/240 ft., one target. Hit: 36 (4d12 + 10) bludgeoning damage.

Frightful Presence. Each creature of the zombie tarrasque's choice within 120 feet of it and aware of it must succeed on a DC 17 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 zombie tarrasque's Frightful Presence for the next 24 hours.

Swallow. The zombie tarrasque 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 zombie tarrasque, and it takes 56 (16d6) necrotic damage at the start of each of the zombie tarrasque's turns.

If the zombie tarrasque takes 50 damage or more on a single turn from a creature inside it, the zombie tarrasque must succeed on a Constitution saving throw at the end of that turn with the DC equal to half the total damage, or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the zombie tarrasque. If the zombie 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 zombie tarrasque 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 zombie tarrasque regains spent legendary actions at the start of its turn.

Attack. The zombie tarrasque makes one claw attack or tail attack.

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

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

Hurl Earth (Costs 2 Actions). The zombie tarrasque makes a hurl earth attack.