Boglurker Greatcrab

Negligible Senescence. Greatcrabs have no maximum lifespan, but their rate of growth slows as they age. Their age category and AC are determined by their rolled HP maximum: 

• Young- 245 or less, 17 AC 
• Adult- 246 to 289, 18 AC
• Venerable- 290 or more, 19 AC

Boglurker Sense. The crab has advantage on perception checks when in aquatic or semiaquatic environments.

Regrowth. The crab can regrow lost legs, claws and HP over the course of 30 days, and recovers twice as fast while buried.

Thick Upper Shell. The crab has +3 AC against attacks that would strike the top of its shell, such as attacks from flying creatures or arcing projectiles at long range. If an effect coming from above allows it to make a Dexterity or Constitution saving throw to halve damage, it instead takes no damage on a success and half on a failure.

Amphibious. The crab can breathe air and water.

Multiattack. The crab makes two attacks with its claws.

Claw Grasp. Melee Weapon Attack: +14 to hit, reach 20 ft., one target. Hit: 18 (2d8 + 9) slashing damage, and the target is grappled (escape DC 24). If the target is Huge or smaller, it is restrained. If it is Large or smaller, it is lifted off the ground and the claw remains free for swipe attacks. If it is already grappled, it has disadvantage on checks to escape the grapple, and is restrained. 

Claw Swipe. Melee Weapon Attack: +14 to hit, Reach 20 ft., one target. Hit: 31 (4d10 + 9) piercing or bludgeoning damage. If the target is reduced to 0 HP,  the crab can deal any remaining damage to another creature in range within 10 feet of the target. This can be repeated for each creature reduced to 0 HP. If the claw contains a grappled creature, it takes 14 (4d6) bludgeoning damage on a hit or miss.

Dig. The crab falls prone and digs itself into soft ground, becoming half buried until it moves. A half buried crab always has the benefits of its Thick Upper Shell. If a half buried crab uses this action, it becomes fully buried 5 feet under the surface. A buried crab is in full cover, cannot attack, and is heavily obscured. If its eyes or antennae are extended above the surface, they can be discerned from the surrounding detritus by a nature check contested by the crab's stealth. 

Dash. The crab can only dash as a bonus action. The crab swims through open water by dashing; otherwise it must walk across the bottom. 

Pinch. The crab clamps down with one or both of its claws. Creatures grappled by the crab take 27 (4d8 + 9) bludgeoning damage from each claw grappling them.

Slam. When a creature moves into or ends its turn in the crab's space, the crab can use its reaction to fall prone. Each creature in the crab's space must make a DC 19 Dexterity saving throw, moving to the closest unoccupied space on a success. On a failed save, the creature takes 82 (12d6 + 40) thunder damage, is knocked prone, has total cover from all attacks, and is paralyzed until the crab either dies, digs, or stands up from prone.

Snatch. A creature that flies within the crab's reach provokes a Claw Grasp attack of opportunity, provided the crab can see it and has a free claw.