Kelpie

In their true form, kelpies are indistinguishable from seaweed. They live in saltwater and freshwater, lakes and swamps, floating water and stagnated pools. Sages and scholars believe kelpies were created by an evil water goddess or an unusually powerful elemental entity. In her human guise, a kelpie appears as a beautiful woman of any humanoid race, with flowing dark hair, emerald eyes, and soft pale skin with a slight greenish tint. She is cloaked in robes of seaweed or wears nothing at all, and has normal humanoid limbs.

Kelpies enjoy luring people to a drowning death. They lie in wait along regular shipping lanes or the banks of rivers and lakes. When a vessel or a shore party approaches, they reshape themselves as women and begin acting as if they're drowning, or floating unconscious on a piece of wreckage from a lost ship. If the ruse is not detected and the observers come to their aid, they try to charm their rescuers. Kelpies aren't interested in fighting to the death; their method is to charm a few victims, lure them into the water, then dive and swim down into deep water while their victims swim down after them and drown.

Traits
Amphibious. A kelpie can breath equally well in air or underwater.

Charm (1/day). As an action, a kelpie can force a humanoid who can see the kelpue or hear her voice to make a DC 13 Wis saving throw. If the save succeeds, the creature is immune to the kelpie's charm for 24 hours. If the save fails, the victim is charmed by the kelpie for 24 hours. A charmed creature percieves the kelpie as a beautiful and alluring creature and wants nothing more than to sit adoringly by its side, even if that means diving underwater and staying there forever. The charm is broken if the kelpie is killed or knocked unconscious, or if dispel magic, lesser restoration, or comparable magic is used on the charmed creature.

Telepathy. A kelpie can communicate telepathically to a range of 1 mile with any creature she's touched who is not immune to her charm.

Actions
Claws. +4 to hit, reach 5 ft; one creature. Hit: 1d6 + 2 slashing damage and a humanoid target must make a successful DC 13 Wis saving throw or be charmed by the kelpie. This effect is the same as the kelpie's charm ability, but a humanoid charmed by this attack can repeat the saving throw at the end of each of its turns. The charm is broken by a successful save. Charming from the claw attack is unlimited.

Multiattack. A kelpie can make 1d4 frond attacks. The amount of attacks is determined each round.

Frond. +4 to hit (reach 15 ft; one creature). Hit: 1d6 +2 bludgeoning damage and the target must make a successful DC 12 DEX saving throw or be grappled, see Strangling, below.

Bonus Actions
Strangling. As a bonus action, a kelpie can strangle one creature that was already grappled at the start of the kelpie's turn. The kelpie drags the grappled creature underwater and wraps 1d6 additional fronds around the target, which becomes grappled and restrained. The DC to escape this grapple equals 10 + 2 per frond restraining the creature. A restrained creature takes 1d8 + 2 bludgeoning damage at the start of each of its turns. Strangling fronds have AC 10 and can be severed by 4 slashing damage each.