If you weren't deliberately stealing ideas from Silent Hill already, I'd recommend you do that; while plagiarizing is a very bad idea when producing content you plan to sell or turn in for a grade, it's a huge time-saver for making home campaigns and settings. I recommend Silent Hill because you've got some major similarities to it already; I'm recommending the movie rather than the games since that only takes 2 hours or so to watch.
Keeping the Players in the Area
There are several choices here, most of which are ultimately going to take one of three forms:
An impassable barrier of some kind. In Silent Hill it's a huge canyon, too deep to see the bottom and too wide to see the far side, so the world seems to end in a misty cliff, but really you can use whatever you want. It could be something as simple as the forest being in a mountain valley, with the mountain slopes being too steep to climb and rockslides having blocked the road at the two entrances to the valley.
This is arguably a subtype of the first option, but you can have something forcefully redirect the players back into the area you want them in. Again, you have lots of choices. Maybe an unkillable monster (it needs to be obviously unkillable) attacks from the edges whenever they start to leave the area, then abandons the chase once they return to the region. You could also use some sort of environmental effect with a gradient, like horrible screeching that gets louder & louder as they push farther out of the area, until the party is forced to turn back.
World wrap. A good choice for constraining an unusual area without barriers, world wrap is also common in strategy & RPG video game world maps.
A. Essentially, when you leave the map on one edge, you appear on some edge of the map heading back in. This could be blatant such as the fog being super thick at the edges, and after 20 or so paces through the fog you emerge right where you entered the fog, but now heading in the opposite direction (i.e. back into the allowed area).
B. Alternately, you can use a more classical toroidal world wrap: leave the area heading north and you re-enter it from the south, leave the area heading west and you re-enter it from the east, etc. Much like the Earth being a sphere, if you pick a direction and stick with it long enough, you'll eventually get back to where you started coming from the direction opposite the one you left in.
Note that having noticeable and identifiable landmarks (or encouraging the players to start creating such things early) is pretty much mandatory if you use any form of world wrap.
You can also check out the Ravenloft setting for D&D for more ideas; each of the Domains there could be sealed by its Lord, with the sealing taking a different form for each Domain.
Giving Plot Information to the Party
I'll just toss a couple ideas out for you to consider, since you don't have any helpful NPCs.
- Helpful NPCs' ghosts
- Diary fragments (is language barrier a
problem? can you convince one of the PCs to be an archaeologist?)
- Elaborate (and disturbing) cave paintings
- Prophetic dreams while asleep and/or prophetic visions (accompanied by seizures) while awake
- Cloud or mist formations that look like something
- Some kind of odd animal(s) repeatedly acting out a simplified version
of what the party needs to do (thanks Malazan Book of the Fallen!)
Note that 1 & 2 are a lot easier if you adjust your backstory so that someone else was grabbed not too long ago (a decade at most) and died.
You should think about the backstory of this setting. How did the psectadron come to exist? Did it come from some other realm of existence? Did someone deliberately turn themselves into this thing? Was it the side effect of something?
Destroying the bodies of the Angh-Zuul is certainly doable. To make it reasonable, the bodies need to be at spots that are identifiable in some way; small cairns of some kind, spectral fires, some kind of guardian creature, etc. As for placement, there are two main options. First you can come up with a backstory for how they died. Was it a running battle? They'll be strung out in a line. Went crazy and killed each other? A few clumps of multiple corpses wherever they were when the madness struck, and lots of them scattered around the area as the remaining ones hunted each other down. Alternately, you could place the bodies to form some sort of symbol (a pentagram is easy & classic).
Finding X MacGuffins also works. This one ties pretty strongly into the psectadron's background. Presumably these objects are some leftovers of however it came to exist or came to be tied to this place, and while it would love to get rid of them it can't; it's unable to interact with or affect these objects in any way.
You can also more directly tie into the backstory, by forcing the party to somehow undo whatever created or called the psectadron in the first place. Perhaps a ritual, requiring apparently mundane objects scattered about the area, so the party has to learn the backstory to understand which objects they need?
If these are one-off characters (or maybe even if they aren't, if your players have the right personalities), perhaps someone ultimately has to sacrifice themselves to get the rest of the party out, either to open a portal of some kind for them to escape, to seal the psectadron somehow, or even to become the psectadron and allow the party to leave (game reference, but I'd be spoiling it for anyone that hadn't played it by revealing which game).
Time limit, type 1: the psectadron has to kill them to permanently capture their spirits, and if they can just stay alive long enough it will lose its hold on them. Obviously this requires the scenario to be more dangerous. You also shouldn't tell them that this is the victory condition, so they'll wander around the area instead of bunkering up somewhere. You would probably have everything fade out and become more dangerous as in option 2.
Time limit, type 2: a portal they can escape through will automatically open, but it will close after a short time and never re-open, so the party has to figure out where it will open, get there, and survive until it opens. As an added bonus, you can have it open right where they started, so if they had just stayed put they'd be fine (movie reference, but as before I won't spoil which movie).
You can also mix and match from the above. Maybe destroying the Angh-Zuul corpses is the first step in the ritual, or they were buried with the MacGuffins. Or the ritual could ultimately lead to a sacrifice of some kind.