Last night, one of my D&D groups cracked open the outer door of a prison containing a demon that has been imprisoned for hundreds of years, with the intention of killing it once and for all. But about halfway through the dungeon, low on resources with one of the PCs having been inches from death, they’ve decided that they should probably retreat for now and come back the next day after recovering. They are aware that there is a danger that the demon could escape and either come after them or cause some other havoc.
But if the PCs do decide to turn back and return the following day, what should the demon do in the meantime?
The players are all pretty new to the game, so it may be that the challenges I put in their way before reaching the demon were just too much for them, given their inexperience.
My original concept for this dungeon was that there were two levels of wards imprisoning the demon. So although I don’t hold my plans sacred anyway, it would be consistent with those plans if either the demon couldn’t leave his prison at all, or if he could only partially manifest outside his prison somehow, such as by harassing the PCs or the region near his prison with some lower level beasties. This is the option I’m leaning toward.
No Free Parking Money 
Either harass them with some lower level beasties, or harass any potentially sympathetic locals to make them feel guilty for cracking it open without finishing the job.
I vote the demon breaks free and causes problems. Players seeing the consequences of their actions in the rest of the world makes the world seem more real/alive.
Update, two months after the fact: The demon didn’t escape, in part because I had already decided (in my own head) that the demon’s imprisonment had a layer of redundancy, but he did send something out after the PCs. This way they weren’t able to re-enter the dungeon with completely replenished resources, and the final fight was still interesting.