I generated this map using Mad Monks of Kwantoom. It felt a little small, and the pool of water in the first room was randomly assigned the magical property of teleportation, so I decided that diving into the pool teleports you to an alternate dimension, where the shrine is present but slightly different, and where the air is filled with sulfurous yellow fog. My players' explorations didn't reveal that detail at the table, but you might have a different experience.
I used the Black Powder, Black Magic vol. 4 zine to fill out the contents of the shrine. "Hezzemuth the Painmistress" is the demon associated with level 1 in the Maw. She's described as having 4 arms and the lower body of a bullet ant. Her shrine is supposed to have traps (random selection led to bear traps this time), and desecrating the shrine has consequences (random selection led to living statues). Rather than having only a chance of a "rare opportunity", I decided there definitely would be one, and I picked (and combined) "A chance to interrupt a human sacrifice." and "A chance to interrupt a demon summoning." rather than rolling randomly. My initial plan was that whenever the characters arrived, they'd have the chance to interrupt the sacrifice, although after they got a direct invitation, if they'd gone to do something else, the sacrifice would have happened in their absence.