Ys (pronounced: Is; ees) or Ville d'Ys is a mythical city built on the coast of Britanny, and later swallowed by the ocean.

A lost city with riches and wealth and fame, swallowed by out of lust.

Legend says Ys was built on a land reclaimed from the sea by the King of Cornouaille, King Gradlon, because his daughter, Princess Dahut (also known in some tales as Ahes) loves the sea.

Ys is described as city rich in commerce and arts. The palace was made up of marble, cedar and gold.

To protect Ys from inundation (meaning: abundance of people) , a dike was built with a gate that only opened for the ships during low tide. There is only one key to the gate. And the key is held by the King.

Other versions of the legend tell that Ys was founded more than 2,000 years before Gradlon's reign in a then-dry location off the current coast of the Bay of Douarnenez, but the Breton coast had slowly given way to the sea so that Ys was under it at each high tide when Gradlon's reign began.

Most versions of the legend present Gradlon as a pious man with a wayward daughter, princess Dahut, who “had made a crown of her vices and taken for her pages the seven capital sins.”
In conclusion, she's kind of a typical rebel teenager.
Except, she's even crazier.

Princess Dahut had a lover for whom she threw a secret banquet and, under the influence of wine, she stole the key to the gate from her father and opened the gate, and the water submerged the entire city.
The lesson here is not to get too high.

Another version of the legend says that she stole the silver key to admit her lover, mistakenly opening the sluices in the dark.
And this one tells us you should never steal and look properly. And not to get too high.

St Gwénnolé, some kind of priest, who, according to one version, had foretold the city's ruin due to its luxury, woke the king and commanded him to flee. He mounted his horse and took his daughter with him. As the water was about to overtake him, a voice called out: “Throw the demon thou carriest into the sea, if thou dost not desire to perish.” Dahut fell from the horse's back, and Gradlon was saved

Though this is the most common version, there's an ancient ballad that blames Gradlon himself for leading his people to extravagances of every kind and says that Dahut received the key from him.

In conclusion, everyone has their own imaginations for this kind of city.

But my favorite version is this supernatural one:

Another version of the legend directly identifies Dahut's lover with the devil.

In other words, for this version, the princess fell in love with the devil. That's creepy and cool.
Maybe the devil would go all Edward Cullen?

Ys was the most beautiful and impressive city in Europe, but quickly became a city of sin under the influence of Dahut. She organized orgies and had the habit of killing her lovers when morning broke. A saint decried the corruption of Ys and warned of God's wrath and punishment, but was ignored by Dahut and the populace.

One day, a knight dressed in red came to Ys. Dahut asked him to come with her, and one night, he agreed. A storm broke out in the middle of the night and the waves could be heard smashing against the gate and the bronze walls. Dahut said to the knight: "Let the storm rage. The gates of the city are strong, and it is King Gradlon, my father, who owns the only key, attached to his neck."

The knight replied: "Your father the king sleeps. You can now easily take his key." Dahut stole the key from her father and gave it to the knight, who was none other than the devil. The devil then opened the gate.

Because the gate was open during storm and at high tide, a wave as high as a mountain collapsed on Ys. King Gradlon and his daughter climbed on Morvarc'h, his magical horse. Saint Winwaloe approached them and told Gradlon: "Push back the demon sitting behind you!"
Gradlon initially refused, but he finally gave in and pushed his daughter into the sea. The sea swallowed Dahut, who became a mermaid.
Well, damn.
Nice kid stories.

Gradlon took refuge in Quimper, which became his new capital. An equestrian statue of Gradlon still stands between the spires of the Cathedral of Saint Corentin in Quimper.
Folklore asserts that the bells of the churches of Ys can still be heard in the calm sea. A legend says that when Paris is swallowed, the city of Ys will rise up from under the waves:Pa vo beuzet Paris, Ec'h adsavo Ker Is (Par-Ismeaning "similar to Ys" in Breton).

So Ys and Paris are like arch-nemesis.

In my version, Ys is indeed in the deep sea of Baie de Douamenez. But in order to get there, you have to pact with special creätures, which Aloin is experiencing.

[source: partly Wikipedia, some in my own words, and partly my own imaginations]