That stupid coffin is leginimately the scariest thing in the entire Magnus Archives podcast. It's not even the fact that you're completely crushed and entombed, but that you cannot die. The one thing you have no matter what—the thing which, no matter how bad things get, promises you an eventual escape—has been taken from you, rendering you truly and entirely trapped. Helpless. Forever. What the fuck. The other entities at least have the common decency to kill you eventually.