Amphibious. The serpentmaw can breathe air and water.
Pack Tactics. The serpentmaw has advantage on an attack roll against a creature if at least one of the serpentmaw's allies is within 5 feet of the creature and the ally isn't incapacitated.
Multiattack. The serpentmaw makes one Bite attack and two Claw attacks.
Bite. Melee Weapon Attack: +5 to hit, reach 15 ft., one target. Hit:10 (2d6 + 3) piercing damage, or 17 (4d6 + 3) piercing damage if the serpentmaw had advantage on the attack roll.
Claw. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit:7 (1d8 + 3) slashing damage.
A scuttling serpentmaw looks like an unusually large crab with a sleek black shell. It lurks in crevasses and deep underwater canyons. Once spotted, it scuttles toward its prey and reveals its most formidable feature: an armored maw concealed on the back of its shell. This serpentine appendage can extend out from the shell to tear into prey.