Amphibious. The crab can breathe air and water.

Ambusher. In the first round of a combat, the creature has advantage on attack rolls against any creature it surprised.

Freedom of Movement. The creature ignores difficult terrain, and magical effects can't reduce its speed or cause it to be restrained. It can spend 5 feet of movement to escape from nonmagical restraints or being grappled.

Reactive. The creature can take one reaction on every turn in combat.

Claw. Melee Weapon Attack: +0 to hit, reach 5 ft., one target. Hit: 1 bludgeoning damage.

multi-attack. The abyssal crab makes two claw attacks. 

Water Whip. Melee Weapon Attack: +12 to hit, reach 25 ft., one target. Hit: 12 (2d4 + 7) slashing damage plus 10 (3d6) cold damage, and the target must succeed on a DC 17 strength saving throw or be pulled up to 20 feet toward the abyssal crab.

Whirlpool (recharge 5-6). Water swirls rapidly around the abyssal crab. Each creature within 20 feet of the abyssal crab must make a DC 17 dexterity saving throw. On a failure, a creature takes 56 (16d6) cold damage and is knocked prone. On a success, a creature takes half the damage and isn’t knocked prone.