I found an adventure online (can't find it now) that was a fairly simple and silly semi-aquatic dungeon delve with a psychic clam as the boss monster. I bumped its stats a bit and gave it aboleth-like ability to create psychic slaves. It built up a group of followers who did not understand its true nature. They believed they were worshiping a god of death, the ocean, and deep mysteries.
It had a nereid minion/psychic slave and would have the nereid shape water to awe the cultists appear as a beautiful woman or man as an angel of their "god". It would regularly lure cultists and their sacrifices into the water, the nereid would use its drowning kiss, and then take the corpse to the giant mollusk, which would consume it. Occasionally, the nereid would present the cultists with wildly valuable, huge baroque pearls that formed around bits of undigestible material of the consumed sacrifices, such as jewelry or weapons. These were treated as relics and the center of worship services for the growing cult.
Of course, the party did not know any of this. They began encountering death cultists while investigating a string of missing persons. Eventually, over time, they learned the secret location of the holiest site of the cultists where the missing persons were taken as (often willing) sacrifices.
The cultists who worshipped the Reaper of the Deep, wore azure robes during their rituals.
During the ritual, the nereid would appear from an impressive display of shaped water and in the form of a stunningly good looking humanoid in the race and gender attractive to the sacrifice. S/he would then sing out words of encouragement:
It is your last night of sadness.
Take my hand.
Do not fear the reaper.
When the party encountered the giant psychic oyster below the waves after defeating the nereid, they realized that all this time they were fighting the Blue Oyster Cult.
