I'm not interested in doing random encounter checks for my AV game; I don't feel like (from what I've read so far--only the first book) that the dungeon nor PF2e really support that style of play.
But I do want to create some kind of time pressure on the players, so taking an exploration turn to search a room, or making repeated checks, or the like, has a cost to it.
Any ideas (especially from those who know the lower levels in books two and three) of what I could make happen at night? It shouldn't be an instant kill type of thing, but something that would chase them out and force them back to town, or at least to the first level. I'm thinking of something like a magical darkness flooding the area.
My first thought is the spell Darkness is cast over the area (heightened so PCs with darkvision are affected as well)?
Is there any reason this wouldn't work, or would create problems later--like they have to do something at night and aren't at a level where overcoming Darkness is more trivial?
There isn't really a point to rushing the PCs, especially at lower levels where they can take 1-4 hours to recover from a single encounter depending on what healing resources they have available. Especially with the town only a 20 minute walk away.
It is disappointing that cautious players will probably never encounter the haunts that only trigger at night, but its also the result of smart plays by them.
Though, I had the opposite problem. Once the beacon lit up the graveyard in chapter two of book 1, they were rushing everything to try to prevent it from happening again. I had to make it clear to them in game that they still had over a month before it would fire again so they'd get back to actually exploring the dungeon.
Though I suppose you have to answer why you want to make time a thing. It sounds like what you want is to have the PCs return to town on a regular basis and you don't really need to do too much time pressure to do that, the natural circumstances of the PCs will take care of it for you.
If you want to incentivize going back to town, the best thing I've found is keep track of encumbrance. PCs pick up so much stuff that unless they're all STR based, they'll get overloaded and have to leave to sell.
A lot of the sidequests require the PCs to go back to get paid. Escorting the thieves guild guys back or getting books for Morlibint. They come across several books they might not be able to read and might want to go back to town to get translated or something.
Keeping track of cooldowns on Medicine also force lower level PCs (At least until someone gets continual recovery) to spend hours regaining HP. Once its dark, enforcing the visibility rules and darkness will make them think twice about constantly staying out and about.
You can also play up how wet and uncomfortable it is on the upper floors. Sleeping in a fen seems silly when you can walk down the road and get some fish & chips at the pub before going to an actual bed.
To give the PCs some sense of urgency, the impending doom from the Gauntlight is great of course, but you could use the Mayor as another factor. I've taken away the possibility for the PCs to find out when the Gauntlight can fire again, to stress them and the town out more, and I instead had the powers in the town set a deadline for the PCs for three weeks to find out what is happening, or they would send the contract on to the Absalom hunting lodges. (Could be fun to prepare a snotty group of adventurers from Absalom that will show up nonetheless and sabotage things.)
As for the dungeon itself, I have no problem adding some wandering monsters if the PCs decide to sleep in there or tarry too long. I have made a small list of some vermin and oozes (I like to keep these unintelligent to not give the PCs any info about the rest of the place, and control the loot) that will start to fill up "unoccupied space" or wander around looking for prey after a while. It is in a spooky swamp after all. Just keep these kind of encounters at about Moderate difficulty and you should be fine.
I you want to have the first couple levels be more dangerous at night a thing you could do is have the necromantic energy in the Gauntlight rooms (B35, C15, D9) fluctuate at night and strike out randomly. Or maybe snake around and chase the PCs in the corridors, dealing its damage per dungeon level (2d6 negative damage for level 2, 4d6 level 3, etc, with differing reflex saves as per the book) to denizens it doesn't like.