Devil Shark

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

Denticles. The devil shark’s skin is covered in razor sharp, tooth-like scales. A creature that touches the devil shark or hits it with a melee attack while within 5 feet of it takes 3 (1d6) slashing damage and 3 (1d6) cold damage.

Magic Resistance. The devil shark has advantage on saving throws against spells and other magical effects.

Magic Weapons. The devil shark's weapon attacks are magical.

Water Breathing. The devil shark can breathe only underwater.

Multiattack. The devil shark makes two attacks: one with its bite, and one with its tail.  It can use its thrash instead of its tail.

Bite. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 29 (4d10 + 7) piercing damage plus 10 (3d6) cold damage.  If the target is Medium or smaller, it is grappled (escape DC 20). Until this grapple ends, the target is restrained, the devil shark can automatically hit the target with its bite, and the devil shark can't make bite attacks against other targets.

Tail. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 18 (2d10 + 7) bludgeoning damage bludgeoning damage plus 10 (3d6) cold damage.  If the target is a creature, it must succeed on a DC 20 Constitution saving throw of be stunned until the end of the devil shark's next turn.

Thrash. Melee Weapon Attack: +10 to hit, reach 5 ft., all creatures in range. Hit: 20 (2d12 + 7) bludgeoning damage plus 10 (3d6) cold damage.