Continuing the series of September Short Adventures (horror themed)... doesn't look like I'll get 25 done in September, but 20 seems achievable.
Squatters and woods folk are disappearing in the Old wood. A forester seeks help to accompany him deep into the Old Wood looking for clues; in the heart of the forest is a hill with a bad reputation, shunned for as long as can be remembered.
SETTING THE SCENE
Identify the appropriate enforcer of the lord's forest-related laws - stopping poachers, chasing out squatters, and collecting taxes - it could be a Warden, or a Sheriff, for instance, that is familiar with the layout of the woods and the various illegal settlements within.
Rumors have reached some of the villages beyond the woods that deep woods squatters, bandits, smugglers, and other dislikable folk have begun disappearing - smugglers miss their pickups, for instance.
Normally the forester would be glad to hear his normal targets are disappearing from the woods - saves him the trouble of keeping tabs on them - but the widespread rumors have him concerned something frightening is actually happening in the depths of the Old Wood. He's authorized to hire escorts in the name of the local Baron and will offer the job to tough but trustworthy adventurers (hopefully the PC's fit that description!)
The forester first sets out for a specific shanty town he knows about near one of the larger muddy rivers that traverses the Old Wood. Over the course of the journey deeper into the woods, one ramshackle settlement after another has been ransacked, everyone gone. It will take a few days walking to reach the heart of the forest. A huge beaten path leads deeper into the woods, making the attacking (humanoids) very easy to follow.
In the center of the woods is Haystack Mountain, the hill with the bad reputation; it juts above the trees and resembles a cone. The hill is like a gigantic ant cone, hollow within, and boiling out of the nest each night like angry ants is a horde consisting of hundreds of subterranean carnivores. I'd suggest something implacable like the Gibberlings from the Fiend Folio.
Every few decades, the nest awakens and for a period of days, the nest of Gibberlings rampages for miles around, stripping the land of animal life and carrying off captives. It's unlikely characters will have time to conduct research before heading out with the forester, but old timers outside the woods might remember the scouring that took place 30 years ago; for a suitably creepy moment, the forester himself is in his 40's, and one night around the campfire retells a story from his childhood about something similar happening.
Folks had once tried to clear sections of the forest and build watchtowers and the like; make sure the group crosses a ruined tower from olden times before reaching Haystack Mountain; you might drop a hint how it could be somewhat defensible, or a good place to camp.
A small group of the Gibberlings (10 or so) got cut off from the horde and are cowering in a nearby shady copse, hiding from the sun. The horses smell them, and are freaked out. If characters investigate, they'll be attacked, and it should be an easy fight. They'll be able to learn the following facts: the tracks of the horde match these creatures; there must be hundreds more of them. The tracks seem to lead back to Haystack Mountain. The creatures were blinded by daylight, and seem nocturnal. It will be dark soon. Did I mention there were 400 of them?
As DM, now it's time to sit back and enjoy the sense of doom and horror that settles on the players as they realize they're deep in the woods (days from a settlement), only a few hours before sundown, and not far from where hundreds of these things will come streaming out of the mountain again. Good times.
On a more serious note, the group needs a PLAN. If they noticed the ruined tower, they might retreat there and spend their last few hours of daylight laying in defenses; I'd love to run this as an overnight siege - all you need to do is make it through the night. The Gibberlings attack in wave tactics, with no apparent strategy other than numbers; they're also afraid of fire, so a group that discovers that they fear flames will have another advantage.
A boat or raft might be an option; fleeing by horse through the woods during night should lead to broken fore legs and lame horses, but I like the image of terrified characters running for their lives (for hours) to get out of the woods. Have fun!
Assuming the players survive and attempt serious research to discover the secrets of Hay Mountain and the reason behind these horrible swarms, here's the story I'd eventually use: the Gibberlings are the children of the "god of the wood", released whenever the god drifts from deep dreaming to awakening, and notices the encroachment of humanity into his sacred places. After a few days of near wakefulness, the god drifts back to deep sleep, and the Gibberlings return to hibernation deep in the hill. Dunno how they'd learn this, but with Commune and Contact Other Plane and sages and whatnot, there's always the chance.
SOME ASSEMBLY REQUIRED
Locale: the Old Wood
NPCs: the forester
Monsters: 400 Gibberlings (AD&D fiend folio)