OK, let's see if I can describe this without confusing you too much.
It's a bit of a wild goose-chase that I made up on the fly, so excuse any glaring shortcomings
Alright, so basically my idea is this: You can do both the small town and the wilderness adventure in one
Say the PCs, on their way home from their previous adventure some across a rather quaint hamlet and decide to stay the night there. The town is all a-buzz and nervous because there have been various sightings recently of a Nightmare in the surrounding woods. The characters, being the good-hearted heros that they are (or greedy opportunists who see a chance to make some extra cash, whichever) decide to try and track the thing down and remove the threat. What the PCs don't know is that it is actually a unicorn, asperi or pegasus (your choice) that has had a permamnent illusion placed on it so that it looks like a nightmare. When they encounter it it will of course behave in a very un-nightmare-like manner (not charging them on sight with murder in it's eyes), which should tip them off that something's amiss. If not and they engage the thing and kill it, the illusion should wear off upon its' death, revealing it's true nature. If however they catch on and communicate with it somehow then they will discover its true nature that way. Either way, it's not sure how the illusion came to pass, and the characters should be inclined to investigate. Back at town the PCs can consult the local wise-woman, who they are told would be the most likely to be able to help them (this is also the same woman the equine victim was trying to find, hence the sightings). When the PCs enquire with her she tells them that the most likely culprit is a certain Night Hag which she knows lairs in a minor bog not a day's journey from the town. Deciding that at the very least they could rid the town of a dire threat, if not put the illusionary culprit to the sword, they decide to go after the night hag.
The night hag, like the rest of her kind, is a merchant of souls from the evil outer planes, and uses a cave in said bog here on the material plane as a storehouse and a sort of "getaway" if you will. The PCs will of course confront her, and that encounter will obviously be resolved in whatever manner they use to approach it, either through diplomacy or mortal combat or anything inbetween. In any case, she is not in fact the one who did it (what possible purpose would that serve her?). She does however have an idea as to what caused it. And she would be willing to help the characters deal with it, because it's caused her a certain ammount of trouble as well. Now of course comes the obvious moral dilemna, kill the foul creature (or try to) and attempt to solve the problem on their own, or accept the help of a not-necessarily-reliable agent of evil?
The problem seems to be a curious (if ambiguous) one emanating from another cave within the bog. It's influence is that it has been causing various random magical effects to spontaniously occur in a certain radius from the cave. Within the cave is a
very long-forgotton shrine to Boccob (or the equivilant god of magic in your campaign), this is the source of the magical out-bursts. The night hag has an elixer that, once poured over the shrine, will nullify it's power. Problem is, the shrine automatically targets anyone within a certain radius of it with random spells (one per crature w/in the radius per round, the spell is determined randomly in a method of your choosing). Furthermore, it is surrounded by a wall of force...
Once the shrine has been dealt with you're free to play the night hag as you see fit. She may hold true to her word and leave, satisfied the threat is over, she may turn on the PCs, wanting to add more souls for her stock, she may turnout to be working on a material-plane beach-head for evil outsiders and, now that the landing point is secure, start summoning demons or devils or yugoloths or what-have-you, she may turn out to be really a celestial in disguise, whatever you want to do. Also, there should be one or two magic items on the shrine for the PCs to snatch up (or fight the night-hag for
).
It's a loose description, I know, but there's plenty of room for you to add or remove whatever you like. Hope you can use it as a fun little aside, good luck!
