So, this time, they have to start at the end and work to the mouth. Of course, it has been re-purposed a bit. There aren't a lot of standing structures and this one, being celestial, appeals to the souls
Fighting through various horrors, they found that the fireplace windows have mantles of blood - by adding blood, they revealed sections of the creature's head - well, from the inside anyway. They managed to find the hinge of the three mandible and make it let go. They set some poison to it, and booked like crazy to escape into the ash storm cover.