Night in the Swamp

Another quick point about wandering monsters in this area. Part of the point is that this isn't just a swamp, but a volcanic, geothermal quagmire. Therefore, some of the monsters should be heat or fire based. Perhaps a few like azers, fire salamanders, fire elementals and half-red dragon veterans. After checking, some of the ones mentioned already are better than I realized, having fire immunities as part of their makeup.
Ideally, the monsters would be mostly traditional to start, with rare instances of fire-based creatures becoming more common the closer the party get to the far side of the swamp and the volcano's slopes. I think elementals and even some demons would be better at the far edge, or after leaving the swamp.
After trying to find their way through the maze of paths, a number of delays forces the party to make camp in the middle of them. The night is particularly dark as a deep fog rises with the setting sun. Steam pours out of thermal vents, and fog rises from the pools of water.
I think a series of "trials" during the night might be fun. Three tests or challenges. First, the haunting of the swamp, with a few undead rising out of the dead pools surrounding the camp. Maybe some will-o'-wisps charm the guard and try drawing them off toward a nearby dead end where two wights come out of the fog to attack.
Secondly, a patrol of azers comes through with a message for a contact they're meant to connect with in the next major canyon, about a mile further along. They are on a mission, and will not stop or tolerate anything interfering with it. If they are defeated (or even if just one is killed with the others getting driven off), the bodies have papers for Sa-peras. There is a congratulatory note on defeating the invaders from the south, and a warning that a party of adventurers appear to have defeated Cragmin in his stronghold, and it appears they have the stone.
Thirdly, the party will be revisited by poor Nalen, now transformed into a blue slaad and rampaging through the swamp, fighting to come to terms with his new identity.
I think having the patrol come first, with the undead fog occurring after midnight, and Nalen arriving in the wee hours of the night if the best way to go.