Abyssal Sight. Magical darkness doesn't impede the mermaid's darkvision.

Amphibious. The abyssal mermaid can breathe air and water.

Dark Devotion. The abyssal mermaid has advantage on saving throws against being charmed or frightened.

Shapechanger. As an action, the abyssal mermaid can shapechange into a medium female humanoid, or back into its true form. If the mermaid dies, it reverts to its true form.

Vile Damage. The abyssal mermaid deals an additional 3 (1d6) psychic damage to creatures it has charmed or frightened

The abyssal mermaid makes a melee attack of her choice.

Abyssal Claws. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 5 (1d6 + 2) piercing damage plus 2 (1d4) necrotic damage.

Hellish Embrace. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 6 (1d8 + 2) necrotic damage and the target is grappled (escape DC 13). The mermaid can grapple only one creature at a time.

Tail Smash. Melee Weapon Attack: +3 to hit, reach 10 ft., one target. Hit: 6 (1d8 + 3) bludgeoning damage and the target must succeed on a DC 13 or be knocked prone. If underwater, the target is pushed 10 feet in the direction of the tail smash.

Demonic Kiss (Grappled Creatures Only). Melee Spell Attack: +4 to hit, reach 5 ft., one target. Hit: 6 (1d6 + 2) necrotic damage and the mermaid recovers a number of hit points equal to the damage dealt. Additionally, the target needs to make a DC 10 Charisma check or be charmed or frightened (mermaid's choice).

Vengeful Wail 2/Day. As a reaction when struck by a melee attack, the abyssal mermaid unleashes a sonic wave of painful energy, dealing 4 (2d4) necrotic damage to the attacker.