Lobstrosity

Amphibious. The Lobstrosity can breathe air and water.

Sense Magic. The Lobstrosity senses magic within 120 feet of it at will. This trait otherwise works like the detect magic spell but isn't itself magical.

Negligent Senescence. The Lobstrosity cannot die of old age.

Multiattack. The Lobstrosity makes two pincer attacks, one with each pincer. If the Lobstrosity is grappling a creature, it can also use its tentacles once. If the Lobstrosity starts its turn while grappling a creature with a pincer, it can use that Pincer to make a Crushing Bones attack instead of a regular pincer attack.

Left Pincer. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 33 (8d6 + 5) bludgeoning damage. The target is grappled (escape DC 16) if it is a Huge or smaller creature and the Lobstrosity doesn't have another creature grappled by this pincer.

Right Pincer. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 33 (8d6 + 5) slashing damage. The target is grappled (escape DC 16) if it is a Huge or smaller creature and the Lobstrosity doesn't have another creature grappled by this pincer.

Tentacles. One creature grappled by the Lobstrosity must succeed on a DC 16 Constitution saving throw or be poisoned for 1 minute. Until this poison ends, the target is paralyzed. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Crushing Bones.  The Lobstrosity can use this attack only when it starts its turn while grappling a creature. One creature grappled by the Lobstrosity must succeed on a DC 16 Strenght saving throw or take 42 (12d6) bludgeoning or slashing damage depending on what pincer was used, or half as much on a successful save. While performing this attack, the Lobstrosity maintains the grapple on the creature.

Pincer Shockwave (Recharge 5-6). The Lobstrosity uses a Pincer that is not grappling a creature to create a Shockwave in a 30-foot cone. Each creature in that area must make a DC 16 Dexterity saving throw, taking 63 (14d8) thunder damage on a failed save, or half as much damage on a successful one. A creature that fails its save is also knocked prone.