Blood Frenzy. The shark has advantage on melee attack rolls against any creature that doesn't have all its hit points.

Water Breathing. The shark can breathe only underwater.

Denticles. The abyssal shark's skin is covered in razor sharp, tooth-like scales. A creature that touches the abyssal shark or hits it with a melee attack while within 5 feet of it takes 6 (1d8+2) slashing damage.

Deep Sea Predator. Abyssal shark's have excellent senses and can detect and follow a blood trail in the water for great distances. They have expertise in perception. 

Bite. Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit: 28 (4d10 + 8) piercing damage.

Thrash (Recharge 6) On a successful bite the abyssal shark can thrash. The target of the bite must make a DC 20 strength saving throw or take an additional 42 (6d10+12) slashing damage on failed save or half as much damage on a successful one. All creatures within 15' of the target must make a DC 20 Dexterity saving throw, taking (4d10 + 8) bludgeoning damage and be knocked prone on a failed save, or half as much damage and not be knocked prone on a successful one.

Legendary Resistance (1/Day). If the abyssal shark fails a saving throw, it can choose to succeed instead.