Cephaloselachi

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

Water Breathing. The cephaloselachi can breathe air and water, but it needs to be submerged at least once a day to avoid suffocating.

Underwater Camouflage. The cephaloselachi has advantage on Dexterity (Stealth) checks made while underwater.

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

Neurotoxic Bite. Melee Attack: +5 to hit, reach 5 ft., 1 target. Hit: 5 (1d4 + 3) piercing damage, and the target must make a DC 11 Constitution saving throw, taking 10 (3d6) poison damage on a failed save, or half as much damage on a successful one.

Tentacle. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 3 bludgeoning damage, and the target is grappled (escape DC 10). Until this grapple ends, the cephaloselachi can't use its tentacles on another target. 

Ink Cloud (Recharges after a Short or Long Rest). A 5-foot-radius cloud of ink extends all around the cephaloselachi if it is underwater. The area is heavily obscured for 1 minute, although a significant current can disperse the ink. After releasing the ink, the cephaloselachi can immediately use the Dash action.