I’m really surprised that there doesn’t seem to be a LARP book, blog or podcast called Ten O’clock Monster.
It’s a common phrase which I’ve heard from gamers and seen in MET and Nordic LARP essays and elsewhere, though I wonder where it originated. It’s pretty self-explanatory - something plot-related comes in and possibly starts a fight a little after halfway through a session where not much else is happening. A plot development engineered with NPCs works as well.
It’s something I was thinking about for the Storytellers Vault MET section - a look at ways to use external threats and different options to liven up a slow game. As a tabletop GM I can use Chandler’s Law whenever I like, but LARPs need people to play the guys coming through the door with guns in their hands, and unless the GM’s doing it they’ll need to be briefed ahead of time.