Is that the same rock from MLM cult thing several chapters back? Don’t remember the exact chapter but it had a kind of crystal displayed in the art gallery. Might be different, it definitely didn’t have the rope around it and this one looks less like a crystal and more like a miniature mountain. Either way, I think it’s safe to say he was lured into the house by that anon to remove the rock. I have no idea how it relates to the family though.
The letter in the beginning I thought was from the phenomenon, an earlier form of it trying to communicate, but looking back it might actually be from one of its victims trying to resist and warn. We get our first real name too I think: “Akira”
I’m also wondering why it’s called House of Bills. I thought it was be an abandoned property filled with receipts for electricity, water, and such.