Ankuram, the Crab Isle

Legendary Resistance (3/day). When Ankuram, the Crab Isle fails a saving throw, it can choose to succeed instead.

Amphibious. Can breathe both air and water.

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

Siege Monster. Ankuram, the Crab Isle deals double damage to objects and structures.

Magic Resistance. Ankuram, the Crab Isle has advantage on saving throws against spells and other magical effects.

Summon Island Denizens (5/day). Ankuram, the Crab Isle can call upon the island denizens to aid it as if casting [spell/Conjure Animals[/spell] at 3rd level.

False Appearance. When remaining motionless, Crab Island is indistinguishable from a normal island. 

Multiattack. Ankuram, the Crab Isle can make two Claw attacks and one bite attack. Ankuram, the Crab Isle can replace Bite with either Swallow or Tidal Wave.

Claw. Melee Weapon Attack: +19 to hit, reach 20 ft., one target. Hit: 18 (4d8 + 10) Bludgeoning damage. 

Bite. Melee Weapon Attack: +19 to hit, reach 5 ft., one target. Hit: 34 (4d12 + 10) Bludgeoning damage. If the target is a creature, it is grappled (DC Dex 20). Until the grapple ends, the target is restrained and Ankuram, the Crab Isle can't use bite on another target.

Swallow. Ankuram, the Crab Isle takes one bite attack against something it is grappling. If it hits, the creature takes the damage and is swallowed. While swallowed, the creature is blinded and restrained, it and has total cover from attacks and others effects outside of Ankuram, the Crab Isle and takes 56 (16d6) Acid damage at the start of the Ankuram, the Crab Isle's turns. If Ankuram, the Crab Isle takes 60 damage in a single turn from a swallowed creature it must make a DC 20 Constitution save or regurgitate all swallowed creatures which land prone within 10 feet of Ankuram, the Crab Isle. If Ankuram, the Crab Isle dies, then any swallowed creatures can use 20 feet of movement to get out.

Tidal Wave (Recharge 5-6). While in the water, Ankuram, the Crab Isle can slam it's claw in the water causing a large wave. The wave is 250 feet long, 100 feet tall, and 25 feet thick, the wave travels 100 feet away from Ankuram, the Crab Isle. Any creature caught in it must make a DC 22 Strength saving throw, or take 33 (6d10) bludgeoning damage and be moved with the wave, on a successful save, take half damage and you aren't moved. 

Ankuram, the Crab Isle has 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. Ankuram, the Crab Isle regains spent legendary actions at the start of their turn.

Move (Costs 1 Action). Ankuram, the Crab Isle can move up to it's speed.

Claw (Costs 1 Action). Ankuram, the Crab Isle can make one Claw attack.

Chomp (Costs 2 Actions). Ankuram, the Crab Isle can use either it's bite or Swallow action.