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

Undead Fortitude. If damage reduces the undead sahuagin to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the undead sahuagin drops to 1 hit point instead.

Undead Amphibious. The undead sahuagin does not need to breath air. 

Multiattack. The sahuagin makes two melee attacks: one with its bite and one with its claws or harpoon.

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

Claws. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 6 (1d8 + 2) slashing damage.

Harpoon. Melee or Ranged Weapon Attack: +4 to hit, reach 5 ft. or range 20/60 ft., one target. Hit: 5 (1d6 + 2) piercing damage, or 6 (1d8 + 2) piercing damage if used with two hands to make a melee attack.