Sepulchre of the Abyss

From the introduction: This religious site is on the abyssal plain at the bottom of the sea and is rumoured to store vast wealth and secrets. Especially devout cultists once went there to prepare themselves for the transition to their deity’s watery domain... through ritual self-mummification. Treasure hunters can enter this place by stepping through a stone portal likely found in a remote coastal cave or an abandoned beach cabin. Such a gate only