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

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

Siege Monster. The Abyssal Gorestalker deals double damage to objects and structures.

Metabolic Regeneration. The Abyssal Gorestalker Regenerates 1D10+10 per living creature it has currently swallowed.

Rampage. When the Abyssal Gorestalker's hit points drop below 50% of its total hit points it attacks with disadvantage but all damage dealt by it is doubled. ( This effect stacks with Siege Monster. ) These effects end when the Abyssal Gorestalker's hit points recover past 50% or when it dies.

Multiattack. The Abyssal Gorestalker 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: +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. If the Abyssal Gorestalker's bite deals damage to a huge or larger living creature it gains temporary hit points equal to half the total damage dealt by the bite.

Claw. Melee Weapon Attack: +19 to hit, reach 20ft., 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.

Javlin Spine Salvo Ranged Weapon Attack: +15 to hit, reach 120ft., one target. Hit: 24 (4d6 + 10) piercing damage. If the target is a creature, it must succeed on a DC 20 Strength saving throw or be pushed back 20ft. If the target creature is pushed within 5ft of a solid surface it is impaled and restrained and may repeat the saving throw at the begining of its next turn to free itself.

Frightful Presence. Each creature of the Abyssal Gorestalker'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 Abyssal Gorestalker 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 Abyssal Gorestalker's Frightful Presence for the next 24 hours.

Swallow. The Abyssal Gorestalker makes one bite attack against a Large or smaller creature. If the attack hits, the target takes the bite’s damage, the target is swallowed. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the Abyssal Gorestalker, and it takes 56 (16d6) acid damage at the start of each of the Abyssal Gorestalker's turns.

If the Abyssal Gorestalker takes 60 damage or more on a single turn from a creature inside it, the Abyssal Gorestalker 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 Abyssal Gorestalker. If the Abyssal Gorestalker dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.

Thrashing Excavation.  Using a bonus action the Abyssal Gorestalker may burrow and move 20ft. Any creature touching a solid surface within 40ft of its path must make a DC 17 Dexterity saving throw or be thrown 10ft in any direction and knocked prone. ( This may only be used if the Abyssal Gorestalker is rampaging. )

The Abyssal Gorestalker 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 tarrasque regains spent legendary actions at the start of its turn.

Attack. The Abyssal Gorestalker makes one claw attack or Javlin Spine Salvo attack.

Move. The Abyssal Gorestalker moves up to half its speed.

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