Looking for a creature halfway between a siren and a hag

[Lupine]

Basically, the idea is that long, long ago, the civilization was struggling to survive: the would was far too dangerous for them to live rurally, but the only land safe enough for civilization also had quite terrible soil. So they either made a deal with a creature, or bound some nature spirit. In either case, every five years, on the winter solstice, the creature kills a man. Usually a fisherman. It takes that man's lifeforce, and rejuvinates the land, allowing the city to feed.
However, since the deal was made, the fact that it was done has been forgotten, and so the players are tasked with finding out why every five years, a man goes missing, and if found is largely skeletized.

The problem is finding a monster that a group of 8 players, all level 4, can reasonably face off against.

I considered both a beefed up Succubus with Legendary Actions and a Legendary Sea hag (coven version). The sea hag doesn't feel right because the creature is supposed to be siren like.
I'm kinda at a loss for what to use here.

Thoughts?

[Sorinth]

At 8 players I'm not sure you should have a single creature as the main fight. Even with Legendary Actions the action economy is very favourable for the players.

Kelpie are Siren like creatures that might fit the story purpose a bit better since they are plants. So perhaps one giant Kelpie and then minions, either smaller Kelpie or other sea creatures that worship/take care of it would work.

[EggKookoo]

Makes me think of a harpy of some kind.

[Lupine]

Kelpie are Siren like creatures that might fit the story purpose a bit better since they are plants. So perhaps one giant Kelpie and then minions, either smaller Kelpie or other sea creatures that worship/take care of it would work.

Funny enough, I actually took "Kelpie" as the creature's name. Didn't know they were a plant creature, but that kinda works.

I think I'm gonna run with a significantly damage nerfed Mummy Lord reskinned for underwater.

[Zilen]

Hi there!

Reading the description for this enemy, it reminds me of the Siren Boss in Darkest Dungeon 1. Basically what you want to achieve in the encounter is for her to mindcontrol someone in your party that should cause the least damage to the rest of your characters. You can check the wiki for this specific boss to get some inspiration!!

Hope it helps!

[da newt]

A coven of Green Hags might be a good option.

1 v 8 fights can be a mess. If you do go that route, legendary actions will be a good idea. The hardest bit to balance is any 1 creature capable of challenging 8 level 4 PCs at all will very likely be powerful enough to kill an individual PC without even trying.