I'm a bit fuzzy on the details, since this was wrapped up over two weeks ago, but the party went into a dark (non-magical) portal rimmed with tooth-like protrusions. Walking through a glistening cylindrical passage, they concluded that whatever they were in, it was probably alive at some point. Pressing on, they found a round chamber filled with water and criss-crossed with wooden bridges. They heard Obed taunting them in the distance, and after much banter the characters were resolved to beat him to death and finally get the fuck out of Greyshore.
Lwaxana used a Water Walking ritual to basically foil most of the hindering terrain for the rest of the adventure, so when tentacles burst from the water to tear it apart, they didn't give two shits and were able to focus entirely on the crauds that were harassing them. After dispatching more crab-people, it was a straight shot to the Obed, who was waiting for them in a larger chamber, also filled with water and wooden platforms. And more crab-people. I remember this being pretty frenetic, with Obed unleashing devastating lighting attacks while his minions screened Hawkeye's ranged attacks and kept Heien occupied in melee.
Eventually, riddled with arrows and bleeding from numerous cuts, Obed erupted into a tentacled horror that tried in vain to get a hold of the silver key that Lwaxana kept on her person. With all of his minions slain the party gang-raped Obed and destroyed another idol dedicated to Dagon as per their usual routine. With Greyshore's cult finally routed, they were able to safely explore the rest of the shrine. They found a portal with an opening that seemed to perfectly accommodate the silver key, but no one in the party would have any of that (pity, really), as well as extensive notes written in Abyssal and Deep Speech.
They packed everything up, went back to Greyshore, fixed up their earth tumbler, hitched a boat, and headed out. Hopefully, the next place they went to wouldn't have a population unanimously dedicated to their end and have some answers to their questions.