It is your dad's WALLET. This is surely the end of the line. Ahead is another crypt. You wonder if he went in there? There's another one of those obelisks nearby shining a light down a pit, serving some dang purpose. Probably has some ridiculous bearing on a puzzle like five miles away. Like activating an elevator or a conveyor belt or something that just leads to yet another stupid old skull.
Actually, looks like there's one more thing left in the wallet. It's a note.