Hey, wanting to make a dnd character based on Lithuanian folklore.

It's a famous story here and one of my favourites, it goes as follows:

One warm summer day, Eglė, the youngest daughter of a local farmer, went bathing in the sea with her two sisters. Afterwards, returning to the shore to get dressed, she found a serpent in her clothes. To her surprise, the serpent spoke to her in a man's voice and demanded that she promise to become his wife for the return of her clothes. Faced with an immediate need to get dressed and not thinking about possible future consequences, Eglė agreed.

Three days later a great number of serpents pulling a wagon showed up at Eglė's parents' farm to claim the bride. Eglė's family tried to trick the serpents by giving them one of their farm animals, but each time a cuckoo warned the serpents about the deception. Finally, wise to the trickery, serpents succeeded in taking Eglė with them to their master.

At the seashore they were met by Žilvinas, a handsome young man, the Serpent King, who took Eglė to a nearby island and then to his palace under the sea, where they wed. Eglė and Žilvinas lived together happily and had three sons, Ažuolas, Uosis and Beržas, and a daughter, Drebulė, their youngest.

One day the children started asking about their mother's former home. Eglė became homesick and asked Žilvinas to allow her and the children to visit her parents' farm. Žilvinas was against it and set a number of what he thought were impossible conditions - to spin a never diminishing amout of silk, to wear out a pair of iron shoes, and to bake a pie without kitchen utensils. Eglė, however, with the help of a local sorceress, was able to accomplish them and Žilvinas had to allow Eglė and the children to go.

The reunion with the family was such a happy event that Eglė's family did not want to let them return to the sea and decided to kill Žilvinas. But first, they needed to know how to get Žilvinas to appear from the sea.They demanded that the children reveal the family secret. Sensing danger, the sons refused to comply, but the youngest daughter Drebulė became frightened and revealed the secret call.

Eglė's twelve brothers rushed to the seashore, called Žilvinas, who appeared in serpent form, and killed him with scythes.

Not knowing her husband's fate and ending her stay with her parents, brothers, and sisters, Eglė returned to the seashore and called Žilvinas. In reply only a bloody foam appeared.

In her grief, after she realized that Žilvinas was dead, Eglė transformed her family into trees - her sons into an oak, an ash and a birch, her daughter into a trembling aspen, and herself into a spruce.

Basically, I'd want to make the serpent, Žilvinas into a character, perhaps he somehow came back to life? Not sure yet, but what kind of class this would fit or how the turning into a snake situation would work either? I'm thinking probably some sort of curse of sorts, race probably would go with a human or some sort of yuan ti definitely.

Any ideas or suggestions for this? I'd love to turn this into a character since I love my countries folklore, but I'd love to hear people's input on this. Honestly, adventuring motivation could be good too.

It gives me the impression of being some sort of Druid, probably Circle of the Land (coast) but I would think Yaun Ti for race would be appropriate. If you look at it from Žilvinas being an Arch Fey originally then maybe he's reincarnated into a mortal form after his death and has to find a way back to the Fey Realm to get his power back/become an arch fey again or maybe he wants to undo his wifes magic and turn his wife and children back into mortal creatures.

Druid definitely makes sense, but you might want to look at warlock too

A Pact of the Chain warlock has the Voice of the Chain Master invocation, which allows them to speak through their familiar in their own voice

Given that they lived under the sea after the wedding, Fathomless would make the most sense for a patron (and allow you to breathe underwater at first level)

Plus, casting Flock of Familiars with a high enough spell slot might give you enough serpents to pull a carriage ;)

I found something that might be useful for a race, its from planeshift Amonkhet which is an Magic the Gathering crossover thing WOTC did, its found here: https://media.wizards.com/2017/downloads/magic/plane-shift_amonkhet.pdf

Have a look in the racial bit for Naga, it'll give you an alternative to Yuan Ti.

I agree with Rob, a druid would probably be your best bet.