This post contains a complete quest that is suitable for any level and can seriously challenge any D&D party in terms of the moral greyzone. While I would recommend it for any DM who enjoys a more serious tone in their campaign, I would perhaps recommend it more for a party that is overly impulsive and careless of consequences. The reason being that if they follow the quest blindly, they will end up committing acts of serious evil.
There will be several pieces of exposition written in italics. You are free to use these if you wish, but keep in mind that I used them as responses to player decisions, not as a form of railroading.
To set the quest up, heavy rain must have fallen for several days. It is also paramount that the quest is performed during a dark night.
The quest is found in any town or village. It can be attained through a town taskboard, word of mouth, or any other method you deem suitable.
The quest giver can be one or several, but for the sake of simplicity, let's say there is one.
The questgiver is a farmer who, for a long time, has had his livestock stolen, often left in gory shreds and is seeking a solution to the problem. The questgiver will tell the party that Nightcrawlers have terrorized the village for generations, stealing food, items and livestock, kidnapping children and killing villagers who enter the nearby woods. He is relieved and elated that a group of adventuerer's have finally shown up to bring peace to the farmlands once and for all.
The village has not had the manpower or resources to deal with the nightcrawlers. But this task has been further complicated by the fact that they are burrow dwellers, living in underground tunnels that would be lethal to enter.
However, the farmer tells them that now is the perfect time to strike, as he estimates that the extreme rain will soon bring the monsters out of their burrows as they begin to flood. He instructs the party to wait for them to surface and then surprise them as they leave. The quest, at the surface, is nothing more than a kill quest. A kill quest, with a big tactical advantage. Easy money.
The grove to which the party must venture is a 20 minute trudge through the dark and the rain. Here is the descriptions I used to set the mood (keeping in mind what I wrote in the introduction):
You venture out of Millstone, with Millbrook Grove in your sights, and follow into a beaten path leading southwards. You walk through the humid blackness, feeling your feet quickly drench in the muddy water below, flashes of lightning illuminating drowning meadows and steep hillsides as you pray that your light source will not abandon you in the dark.
As you pass by a wheat field, another flash erupts, and you see a figure standing in the middle of the field, staring straight at you. You can barely register the sight before the dark returns, leaving only an afterimage of the figure. [On further inspection, it's nothing but a scarecrow.]
You walk onwards, eventually coming to a thickening of the flora, a dense forest starting by your feet. You manage to find another beaten path into the Grove.
Now arriving at the forest, the adventurers soon come to the target area:
You eventually come into a clearing in the Grove, and as another strike of lightning flashes up the area in a blinding white light, you see a number of mounds in the earth before you.
[Upon entering the clearing]: Stepping closer, you stop at the first mound. You find that it has a hole, large enough for a small human to fit inside, but not much more.
[Upon inspecting the holes]:Inspecting the other mounds, you find that there appears to be a total of five of them, spanning a radius not much more than sixty feet. This must be the burrow.
The task is now simple. They have found the lair of the nightcrawlers, and unless they're too late, the monsters will soon surface. All they have to do is wait.
If the party chooses to wait in ambush, the enemy soon surfaces. Here is how it played out in my party. Keep in mind, they were quite blind in the dark:
Your ears twitch as you hear sound coming from a nearby hole. It's faint, distant, yet a sound was definitely made.
[On waiting]: The sound comes closer, low grunts and the shuffling of mud. You feel your hearts in your throats as you prepare for whatever may emerge, weapons in hand.
[On waiting]: Suddenly, a head emerges from below, trying to push itself to the surface.
You can have the players roll for hit and damage, but make whatever they are hitting weak enough to almost entirely guarantee one-hit-kills. The following expositions of course depend on weapon type. The key is to describe it in as vivid detail as possible.
[On immediately attacking]: You smash into the creature with all of your might, and you hear the weapon make contact with the creature's skull, breaking it with a crunch. Its body begins to slump back down into the tunnel, but another seems to be pushing it upwards. A mere second passes before another head can be seen, the one just slayed pushed onto the mud.
[On continuing to attack]: Once more, a deadly thunk is heard as the weapon aims straight for the head, gutteral voices responding from below in a language you cannot understand and this body slides back down into the hole, and you hear a splash from below. The tunnel seems to be almost entirely flooded.
[On continuing to attack]: A third head emerges as another desperately tries to push its way to the surface, and PLAYER, you feel something claw onto your leg for leverage.
[On pushing away the clutching claw]: You wrest your leg free as the creature slips back down into the hole. You hear frantic shuffling from below as it tries to grab onto whatever or whoever it can find, before you hear something heavy plunge into water.*
From behind you, you hear another noise, and as you quickly turn your attention towards the back, you see a creature begin to emerge from a different hole.
The first twist should be quite clear by now to anyone reading this. The party currently engaged in a slaughter of innocents, they just don't know it yet. They are not, in fact, bloodthirsty demons, but a local kobold population. Unless your party took precautions to prevent such a massacre, they will have already killed a few defenseless kobolds trying to escape death by drowning below. Make the desperation and frenzy below as vivid as possible before the reveal. Make the party feel powerful. Once you feel like your party has done enough damage to make the twist sting, there are a few ways you can reveal it:
In reality, the kobolds have indeed been quite a nuisance, stealing chickens and scaring daring children throughout the years, but the stories of bloodthirst nightcrawlers are merely concotions of collective paranoia and urban legend.
How you continue from here depends on what your party does, but here is where it gets interesting.
Unbeknownst to the players, rumor of their task began to circulate after their departure, and a dozen villagers have found some drunken courage to assist the players, to take up their torches and pitchforks and march off to the grove themselves.
At this point in the quest, it is very likely that your party will have stopped what they are doing, realizing that they have been mislead into performing a massacre. Perhaps they have begun helping the kobolds evacuate their flooded burrows. This is where the party begins to hear shuffling and mummering from behind them, finding an angry mob of locals ready to deliver the final deathblow to their supposed terrorizers, standing by the clearing in dim torchlight. They reek of alcohol.
The party must now choose. Do they side with the locals and continue the extermination? Or do they side with the kobolds, defending them?
No matter the choice, the consequences will be dire. If they side with the villagers, innocent blood will be on their hands forever, having participated in a cold-blooded massacre brought on by stupidity, ignorance and paranoia. If they choose to side with the kobolds, the players lose all promise of reward, and the village will consider them cowards and weaklings, a rumor which might spread and land a serious blow to their reputations.
This is one of those quests that can go either way, at many points in the questline and is therefore flexible and open to improvisation. If However, if executed correctly, in such a way that the party is convinced that the monsters they're going to slay are actually monsters and that they're carrying out a routine deed of good for the village, they'll soon find themselves with the blood of innocents on their hands and it'll be too late to undo what they've done. Even if built up perfectly, the party might after all understand that village folk are paranoid and superstitious and will enter into the quest with trepidation. If that were to happen and not one drop of blood is spilled, they will still get to experience the second twist.
[Keeping this open to edits in case of feedback]
Darkvision is an obvious problem that might make this quest difficult to pull off. I nerfed darkvision at the start of my campaign for these kinds of reasons. If necessary, make the heavy rain another layer of visual obfuscation before the first twist is revealed.