Chauliodus

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

Limited Amphibiousness. The Chauliodus can breathe air and water, but it needs to be submerged at least once every 4 hours to avoid suffocating.

Bioluminescent. The Chauliodus has a set of bioluminescent ridges on its body which give off 5 feet of dim light. 

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

Mimicry. The Chauliodus can mimic any sounds it has heard, including voices. A creature that hears the sounds can tell they are imitations with a successful DC 14 Wisdom insight check.

Undetectable. In the pitch black waters or darkness, the Chauliodus can shut off all of its bioluminescent lights and become undetectable with any sight or hearing. It automatically passes any stealth check when in pitch black waters or shadows. 

Sunlight Sensitivity. While in sunlight, the Chauliodus has disadvantage on attack rolls, as well as on Wisdom perception checks that rely on sight.

Multiattack. The sahuagin makes three melee attacks: one with its bite and two with its hand axe.

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 5 (1d6 + 2) piercing damage.

Hand Axe. Melee or Ranged Weapon Attack: +4 to hit, reach 5 ft. or range 20/60 ft., one target. Hit: 5 (1d6 + 2) slashing damage.