Rusalka

Amphibious. The Rusalka can breathe air and water.

Liquid Form. The Rusalka can enter a hostile creature's space and stop there. She can move through a space as narrow as 1 inch wide without squeezing.

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

Multiattack. The Rusalka makes two attacks: one with her Tangling Hair and one with her Deathly Touch.

Tangling Hair. Melee Weapon Attack: +5 to hit, reach 10 ft., one target. Hit: 7 (1d8 + 3) bludgeoning damage. 
The target is Grappled (escape DC 13). Until this grapple ends, the target is Restrained, and the Rusalka cannot use her hair on another target.

Chilling Touch. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 3) cold damage.

Drowning Kiss (Recharge 5–6). The Rusalka kisses a creature she is grappling or who is Incapacitated. The target must make a DC 13 Constitution saving throw. 
On a Failure: The target takes 21 (6d6) necrotic damage and begins to suffocate immediately.
On a Success: The target takes half damage and does not begin suffocating.

Sorrowful Melody (1/Day). The Rusalka sings a magical melody. Every humanoid within 300 feet of the Rusalka that can hear the song must succeed on a DC 13 Wisdom saving throw or be Charmed until the song ends. It can stop singing at any time. The song ends if the Rusalka dies or is Incapacitated.

While charmed by the Rusalka, a target is Incapacitated and ignores all other creatures. If the charmed target is more than 5 feet away from the Rusalka, the target must move on its turn toward the Rusalka by the most direct route, trying to get within 5 feet. This movement can trigger opportunity attacks, but before moving into damaging terrain (like deep water if it can't swim), it can repeat the saving throw.

Watery Step. When a creature the Rusalka can see makes an attack roll against her, she can dissolve into the water and reappear up to 30 feet away in an unoccupied space that contains water. This causes the attack to miss.