Chull

Amphibious. The chull can breathe air and water.

Shell Defense. The chull can drop prone, reducing its speed to 0 and increasing its AC by 4. It can take no Actions except to exit its shell and stand, and it can take no reactions or bonus actions.

Powerful Build. The chull counts as one size larger when determining its carrying capacity and the weight it can push, drag, or lift.

Facade. When the chull is motionless, it looks like a boulder. A DC 16 Perception check can determine it's true nature.

Multiattack. The chull makes two attacks with its claws OR uses its crush ability.

Claw. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 6 (1d6 + 3) bludgeoning damage, and the target is grappled unless it succeeds a DC 13 Strength saving throw. The chull can only grapple one target at a time.

Crush. The chull attempts to crush a creature it has grappled. Unless the creature succeeds at a DC 15 Strength saving throw, it take 10(2d6 + 4) bludgeoning damage.