I was playing Fear and Hunger 2 and died in a fake safe room (rip 2 hours, should've known that moldy bed was a trap) , and thought it would be a neat idea to add.
Basically the way it could be incorporated is a creepy safe room that has a bed, and all tiles besides a single tile where you interact with the bed are blue, but that tile next to bed is green. Since it is a single tile, I doubt anyone would be able to get into combat since it is step based to trigger a fight, but the fact that the watch changes from blue to green should warn the player something is up.
Resting and relaxing won't do anything different, but trying to sleep would trigger an "ambush" like Mal0. I'll leave the entity to your imagination, but I could see something like a ghost, sleep paralysis demon, a nightmare, or a living doll being the ambusher. Maybe you'd get an item like with Mal0 that would allow them to follow you from the safe room to any other bed.
Also, if you want to take this idea and run it, you could easily add it into either a way later update or a delve. Or don't, that's also a much easier option.