Cloyster

Hold Breath. While out of water, the Cloyster can hold its breath for 1 hour.

Water Breathing. The Cloyster can breathe only underwater.

Shell Armor. At the start of each of its turns, the Cloyster deals 6 (1d12) piercing damage to any creature grappling it.

Multiattack. The Cloyster fires three Icicle Spears.

Clamp. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 14 (2d10 + 3) bludgeoning damage. If the target is a creature, it is grappled (escape DC 15). Until this grapple ends, the target is restrained, and the Cloyster can't clamp another target.

Icicle Spear. Melee Weapon Attack: +6 to hit, reach 5 ft. or range 40/60. Hit: 6 (1d6 + 3) piercing damage plus 3 (1d6) cold damage.

Spikes (2/day). The Cloyster releases a burst of spiky shards of its shell on the ground in a 30-foot cone. Each creature in that area when the shards are scattered must make a DC 13 Dexterity saving throw, taking 13 (2d12) piercing damage on a failed save or half as much damage on a successful one. Any creature that enters the area or moves through it on its turn must succeed on a DC 15 Dexterity saving throw or stop moving and take 1 piercing damage. Until the creature regains at least 1 hit point, its walking speed is reduced by 10 feet. A creature moving through the area at half speed doesn't need to make the saving throw.

Withdraw. The Cloyster withdraws its body into its hard shell and closes it tight. Any creature that the Cloyster is grappling is released from the grapple. Until it emerges, it gains a +2 bonus to AC, it gains immunity to bludgeoning, piercing, and slashing damage from non-magical attacks, and it has advantage on Str and Con saving throws. While in its shell, it is prone, its speed is 0 and can't be increased, it has disadvantage on Dex saving throws, it is blinded and deafened, it can't take reactions, and the only action it can take are Spikes and a bonus action to emerge from its shell.