The Sea Elves

In ages past, before the coming of man into Arlyss, there lived a subset of Elves in the deep seas of the world. Like the wood elves and high elves of the surface, the sea elves colonized the seas in the name of the Fey, thanks to their natural ability to breathe and thrive underwater. They built massive cities built into the rocks and cliffs of the oceans and reigned over the seas for millennia. They allied themselves with the Tritons to beat back the Sahuagin, who were driven into the deepest recesses of the sea.
Their connection with nature was more elemental than that of their surface cousins, and eventually they discovered the means to manipulate energy from the elemental planes surrounding the material world and its echoes (The Fey and Shadow Realms). They hoped to use this magic to open the way back to their home in the Fey, but their efforts proved disastrous. They opened rifts to a place they called "the deep realm," from which monstrous tentacles creatures with psionic abilities poured forth. The Aboleths and Krakens of the deep realm ensnared the minds of the Sahuagin and, bolstered by their newfound allies, the ancient enemies of the Sea Elves returned to wipe them out and claim the seas for their new masters.
War between the sea elves, their triton allies and the Sahuagin raged on for centuries until one last bastion remained. In the spiraling towers of Dinas Disgleirdeb, the last king of the sea elves Faen’Rouh built a tomb for himself and sealed his subjects within. When the Sahuagin found their way inside, they found the king dead and entombed in the central chamber. His followers fought to their last, and the sea elves were no more. For the Sahuagin, the tomb became a sacred place guarded by vigilant warriors and priestesses to this day.
While the seas have largely remained unexplored, brave adventurers seeking treasures in the deep have discovered ruins of the sea elves often guarded by the surviving Tritons or occupied by Sahuagin communities, neither of which suffer trespassers to live.