Crabzilla

Amphibious. The crab can breathe air and water.

Scaly Shell. The crab is immune to attacks directed at areas other than the fleshy front or the eyestalks.

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

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

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

Incredible Speed. The crab doesn't provoke opportunity attacks when it moves.

King of the Deep. The crab does not take any damage from being in a void or an extremely dense atmosphere.

Long Legs. The crab is able to move over large or smaller creature's or object's spaces as if it was normal terrain, this functions for non-magical difficult terrain as well.

Shock Absorption. The crab takes no damage from falls of a distance under 1000 ft.

Multiattack. When the crab choses an attack using the left or right claw, they may then make an attack with the opposite claw as well.

Right Claw Grab. Melee Weapon Attack: +10 to hit, reach 20 ft.,  one target. Hit: 17 (2d6 + 10) bludgeoning damage. If the target is grappled by this claw ignore the attack roll and only deal the damage. If the target is a medium or smaller object or creature the target is grappled, restrained, and incapacitated. The target or targets may make a DC 25 strength saving throw to escape the claw.

Right Claw Throw. Any creature or object in the crab's right claw moves 500 ft. (2d4 * 100) in a direction the crab choses. When a creature propelled this way hits a wall or another creature, both take 4 + 4*CreatureWeight/100 (1d8+(CreatureWeight/100)d8) 

Left Claw Grab. Melee Weapon Attack: +10 to hit, reach 40 ft., 20 by 5 foot line with all tiles in range. Hit: 31 (6d6 + 10) bludgeoning damage. If the target is a large of smaller object or creature the target is grappled, restrained, and incapacitated. The target or targets may make a DC 25 strength saving throw to escape the claw.

Left Claw Throw. Any creature or object in the crab's left claw moves 500 ft. (2d4 * 100) in a direction the crab choses. When a creature propelled this way hits a wall or another creature, both take 4 + 4*CreatureWeight/100 (1d8+(CreatureWeight/100)d8) 

Jump. The crab instantly jumps 200 feet into the air and up to 50 feet horizontally. Hit: 70 (20d6) bludgeoning damage if the crab lands or hits someone while jumping. After the crab falls everyone within 20 ft of the crab must make a DC 20 Constitution saving throw or be pushed to 25 feet away from the crab and take 35 (10d6) force damage on a failed save or take 18 (5d6) force damage on a successful save.

Burrow. The crab can burrow up to 50 ft. into the ground if it's sand and 20 ft. if it's dirt. the crab gains full cover from burrowing in sand and three fourths cover from burrowing into dirt, as well as a bonus of +5 AC.

Squeeze. Before making an attack action, the crab may chose to deal 17 (2d6 + 10) damage to anything in the right claw and 31 (6d6+10) to anything in the left.

Stomp. The crab may chose to rapidly stomp creatures or objects beneath it, forcing them to do a DC 25 Dexterity saving throw, on a failed save the creatures or objects takes 17 (5d6) piercing damage and becomes [condition]grappled[condition/], and [condition]restrained[condition/], on a successful save the creatures take no damage.

Retract. Upon a creature attempting to hit on of its' eyestalks the crab may attempt a Dexterity saving throw against the creatures to hit. If the saving throw succeeds the crab takes no damage from that attack, and its' eyestalk is buried in its' shell.

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

Jump (Costs 3 Actions). The crab instantly jumps 200 feet into the air and up to 50 feet horizontally. Hit: 70 (20d6) bludgeoning damage if the crab lands or hits someone while jumping. After the crab falls everyone within 20 ft of the crab must make a DC 20 Constitution saving throw or be pushed to 25 feet away from the crab and take 35 (10d6) force damage on a failed save or take 18 (5d6) force damage on a successful save.

Right Claw Throw. Any creature or object in the crab's right claw moves 500 ft. (2d4 * 100) in a direction the crab choses. When a creature propelled this way hits a wall or another creature, both take 4 + 4*CreatureWeight/100 (1d8+(CreatureWeight/100)d8) 

Left Claw Throw. Any creature or object in the crab's left claw moves 500 ft. (2d4 * 100) in a direction the crab choses. When a creature propelled this way hits a wall or another creature, both take 4 + 4*CreatureWeight/100 (1d8+(CreatureWeight/100)d8) 

Move. The crab can move up to its' speed.