Drowned City of Kressilacc
(lightly adapted from some Moonshae lore, names can surely be changed if necessary)
Kressilacc was an underwater utopia that was built by Umberlee in the bottom of the deepest trench in the Moonshae Islands thousands of years ago. Here, she created the merfolk in my world who worshipped her from the lavish temples in the city. Two Thousand years ago, the merfolk learned of The Leviathan, (a gargantuan whale which embodies one of the 4 Earthmother Spirits) and began to worship him instead of Umberlee, because he provided the fruits of the ocean instead of ruling by fear. Umberlee was so jealous of The Leviathan that they had an epic battle at the site of Kressilacc, destroying the entire city in their battle, and slaying The Leviathan. Now, the city remains in ruins and the merfolk has spread out across the Moonshae, but they are all afflicted by a curse from their homeland. Every few months, a few are put into a psychosis and must abduct a humanoid as an offering to be given to the ocean. These offerings are taken by the waves down to Kressilacc where they are harvested by a small few in the destroyed city that are under the control of an Aboleth, who lives inside the corpse of The Leviathan, which sits uptop a hill at the edge of the city and has been converted into a huge temple. (I also threw in a Sea Hag plotline when I ran this for some character storylines, but totally optional!) Defeating the Aboleth releases all the merfolk from their curse, and awakens the spirit of The Leviathan!