The Glimmering Sea

The Glimmering Sea is a massive underground cavern in the Upperdark, It is almost entirely flooded over by a giant sea called the Glimmering Sea. It's called this because on the roof of the cavern lies worthless gemstones that provide a very dim light. However, when the light reflects on the water below it the sea looks like an ocean of stars. In the middle of the ocean is an island that has a 5000-foot-tall statue of a dragon on it, this statue is known as Great Stonton. In the shadow of the massive statue lies The City of Shadows. Little is known about Great Stanton, but it is protected by some kind of abjuration magic to prevent any damage. The Dragon statue is of the incredibly rare Prismatic Dragon. On one end of the cavern is a massive wall of water that cascades into the cavern from the Silent Sea. This is known as The Infinite Waterfall, it is incredibly dangerous because of the force of the waterfall. Occasionally creatures from outside the cavern get flooded in through the Infinite Waterfall and live. This was considered to be a non-issue by the denizens of the cavern until a Kracken tumbled through in the year 1307 consistently attacking villages and cities whenever it grew hungry. In the cavern are the Isles of the Lost a massive maze of water and stone that is nearly impossible to navigate. However, there is a rumor that an old god hid his treasure at the center of the Isle. This occasionally makes denizens of the cavern attempt to navigate the Isles, so far none have returned. While there are paths to the Silent sea in the cavern they are incredibly dangerous, as the Twin Dragons guard them. The two black dragons are Derovidod and Fivnel, they guard the only two exits by sea making them impossible to traverse. Only a few times in history have those from the cavern managed to get past them, and even past them are incredibly dangerous networks of fast-moving underwater caves until they eventually reach the empty ocean. Because of the near impossibility of escape, most are content living in their small world. There is a third exit to the cavern that is by land but this one is the most dangerous by far. It is located in the far back of the city Dealvon, Dealvon also known as the city of Tyrants is a city of beholders that are commanded by the Elder Orb Yethmayadar. Its population is estimated to be around 25 beholders as well as hundreds of Spectators and Gazers. Yethmayadar knows the position it's in and guards the only land exit to the cave fiercely. In the Glimmering Sea, there are several cities belonging to Aquatic Races that are underneath the sea. There are also some cities that belong to other Underdark residents. War is a constant in the cavern because many ancestral enemies dwell in the cramped cave, constantly fighting for land and resources. There is no wood in the cave so often times bone and coral are used for construction.
The underwater cities in the cavern are: Xani the Aboleth Outpost, Xxiphu the city of the Aboleths, Trelvo the city of Tritons, Benshilmondoplop the city of Kuo Toa, Ixitachitl the village of Ichaxinogob, Last Hope, the village of Locathah, Mavaka the city of Merfolk, Murcccc the city of Aldani, Velran the city of Sea elves, Wisabid the Capital of the Sahuagins and Dankle another Sahuagin village.
The above water cites in the cavern are: Limelight the Duregar Fortress, The City of Stars the Drow city, The City of the Crashing Waves, which is resident to a large mix of surface-dwelling races. Fading Light, the village of Githyanki, Uraguia the city of Githzerai, The city of Shadow which is resident to hundreds of Driders, and of course Dealvon The city of Tyrants resident to Beholders.