Help me expand on this one-shot idea

[minute]

I'm not sure if this is the best place to post this but whatever.

I'm a fairly new DM with only a couple of one-shots under my belt and recently I've been wanting to develop a horror themed one-shot. The initial idea I had was a sort of haunted mansion type deal, where living inside the house is a bunch of cultists/slaves, and underneath the mansion is actually a large underground lake, where an aboleth has been residing for many years. The individuals inside the mansion are enslaved by the aboleth, and they've been instructed to lure some of the town's commonfolk into the mansion so that the aboleth can enslave them. I figured the party would enter the mansion having been invited to dinner by the residents of said mansion. The dinner would be super creepy and all that (I'd get to "flex" my roleplaying chops here by playing some creepy/sinister folk), and the residents would eventually attack the party, trying to bring them to the lake underneath the house. That would be the first fight, which would be followed essentially by some exploration of the mansion and puzzle solving (I'd definitely take some inspiration from the Death House adventure for this part) and the final fight would be when they finally make it downstairs and encounter the aboleth.

If you couldn't already tell, this is a very unrefined idea. I have all of these pieces in my head but I'm struggling to put them all together. Why would the party want to enter the mansion in the first place, even if they were invited? What's to keep them from leaving the mansion immediately after the first fight when they realize something sinister is going on? Would the party have heard rumors about what goes on at this mansion before entering? Does it make more sense for them to have been hired to investigate this mansion, instead of being invited in? These are all a bunch of the questions that are floating around in my head that I can't seem to find a good enough answer for. I feel like I have a really interesting idea here that will really allow me to practice my "horror DMing", but once again, I'm struggling to put it all together. I will say that I think it'd be super ominous and fun if, when the party eventually defeats the aboleth and exits the mansion, they see essentially the entire town standing, ominously and menacingly, outside the front door. The assumption here is that there are actually multiple aboleths who have infiltrated this town, and the party has only defeated one of them. The session would end there on a cliffhanger, leaving it open for a part 2 if I ever wanted to do that. This is definitely not set in stone though, just a cool idea for an ending that I have!

Let me know if you guys have any advice! I want it to be around tier 2/3, probably around levels 7-10, and I'm willing to adjust some stat blocks to meet that requirement if necessary. Thank you!

[Nidgit]

One possibility would be to include another NPC guest trying to investigate the mansion that leads the party upstairs and clues them in on some of what's going on. They can mention the reward they were offered as extra incentive for the party to investigate and even be kidnapped and taken to the lake to give extra urgency for the descent to the lake.

Alternatively, said NPC could already be mind-controled and just be tasked with luring the party deeper into the estate instead of allowing them to escape.

[Darth Credence]

I think this shows a lot of promise for a one shot. I would probably take a few cues from House on Haunted Hill, where the owner of a haunted house invites a bunch of different people to spend the night, with anyone who makes it all night getting a monetary reward. The movie version of Clue might also be worth looking at for some inspiration. If you don't want them to know it is haunted before hand, maybe you have everyone get invited to a reading of a will, and they are told they get their cut in the morning as long as they don't leave.

As a one shot, this will allow you to have characters that don't know each other before the game, which can add an extra layer of tension to your haunted house. You can even have everyone with a different, hidden reason why they go to the party. Say everyone is aware of the monetary prize, but they each also have some way they are being blackmailed or threatened to come show up. Even without that, the gold reward should be good for getting people to stay longer than they would if they were just invited for dinner and things got weird. If you want the players to have tension between them, I'd make the prize for staying all night be split between everyone who makes it.

I'd say there should certainly be rumors, and I might give everyone some individual information about what they had heard that they are free to share or not. Maybe some have heard that the owner is crazy but harmless, others have heard it is owned by an evil wizard, or even some have heard outlandish, unbelievable stories about how an ancient evil lurks beneath the mansion and eats the souls of those foolish enough to venture in.

I might come up with some type of magic that erects a wall of force surrounding the building at night, so once the sun has set, it is impossible to leave until morning. I love the idea of stumbling out of the house at dawn, bloody and injured from the night inside, to see the entire town waiting for them. That would really mess with the players, in a great way! I think this is a great idea, and I may have to steal it for a one shot myself.

[minute]

I think this shows a lot of promise for a one shot. I would probably take a few cues from House on Haunted Hill, where the owner of a haunted house invites a bunch of different people to spend the night, with anyone who makes it all night getting a monetary reward. The movie version of Clue might also be worth looking at for some inspiration. If you don't want them to know it is haunted before hand, maybe you have everyone get invited to a reading of a will, and they are told they get their cut in the morning as long as they don't leave.

As a one shot, this will allow you to have characters that don't know each other before the game, which can add an extra layer of tension to your haunted house. You can even have everyone with a different, hidden reason why they go to the party. Say everyone is aware of the monetary prize, but they each also have some way they are being blackmailed or threatened to come show up. Even without that, the gold reward should be good for getting people to stay longer than they would if they were just invited for dinner and things got weird. If you want the players to have tension between them, I'd make the prize for staying all night be split between everyone who makes it.

I'd say there should certainly be rumors, and I might give everyone some individual information about what they had heard that they are free to share or not. Maybe some have heard that the owner is crazy but harmless, others have heard it is owned by an evil wizard, or even some have heard outlandish, unbelievable stories about how an ancient evil lurks beneath the mansion and eats the souls of those foolish enough to venture in.

I might come up with some type of magic that erects a wall of force surrounding the building at night, so once the sun has set, it is impossible to leave until morning. I love the idea of stumbling out of the house at dawn, bloody and injured from the night inside, to see the entire town waiting for them. That would really mess with the players, in a great way! I think this is a great idea, and I may have to steal it for a one shot myself.
Thank you for the ideas! I was actually thinking of having it so the party all knew each other already (just a typical adventuring party), but making them all be strangers is a fantastic idea. It would add a whole lot more tension not being able to trust even your own party members, and since I plan to lean very heavily into the ominous horror element of it all, this would work perfectly. The group I plan to run this for is also very into RP, so sitting there and watching them all argue in character would just add to the fun.

I like that wall of force idea. I was also thinking of having all the entrances/exits to the house have an arcane lock on them. Since you can obviously still bypass an arcane lock with a knock spell or just a super high thieves' tools check I'd probably have to modify it a bit, but your idea would make it much simpler! And I'm honored to see you say you might wanna steal this idea haha. I'm still very new to DMing, especially coming up with my own adventures, so that means a lot!

[Kurt Kurageous]

Check out Death House, which I think is a free PDF. It is a pre-adventure from the critics favorite published Curse of Strahd.

It almost perfectly tracks with your ideas.

[Catullus64]

Love the setup, but I would consider re-working it to be a lower-level adventure, Level 4 at the highest. Even the most disturbing and psychologically devious horror loses something when your heroes can take a knife in the back without sweating, and can cast stuff like Wall of Fire or Death Ward, to say nothing of Raise Dead. At low levels, the characters are extremely vulnerable.

The only new problem this creates is that your players are no longer capable of realistically fighting the Aboleth, to which I say... good? Horror adventures benefit, in my opinion, from an enemy too powerful and terrifying to be fought directly. You can still design an action climax, instead of fighting the Aboleth, can be fighting off the cultist slaves in order to reactivate the Six Seals which will imprison it, or to find the Flute of Wat'Yvr that will lull it to slumber. And during that fight, the Aboleth can send nightmare visions or something to disrupt them and help things feel extra nightmarish.

And if the players ignore your signposting and try to take the Aboleth on directly... they die. Or get enslaved. Or some of them die or get enslaved, leaving the others to flee raving and screaming into the night. It's a horror one-shot, is it really a problem if that's how it ends?

[kingcheesepants]

This is a great outline for a horror game (though if you're planning on actually finishing this in one 3-4 hour session it might be a bit rushed, this looks like 2-3 sessions). I'd say to add to the horror element, don't have all the townspeople and other residents mind controlled. Have them in the beginning stages of mind control, have them slowly losing their sense of identity and ability to decide things for themselves in front of the player characters and then have the players experience this same feeling. When the butler or whoever brings them into the house let the players discover him wracked with guilt and muttering to himself "you promised never again", if the players confront him about this he'll instantly compose himself and say nothing is wrong (in an almost supernaturally calm and composed voice) but then when the players turn to go he manages to mutter out a whispered "run" (in an obviously more authentic voice). Have some of the residents complain of strange nightmares, and asking the players to repeat themselves (when the players didn't say anything (the residents are hearing the aboleth's psychic whisperings). Have them do inexplicable things and then look around in confusion once they're snapped back to themselves. And then have the players experience the same. The nightmares, the whispering, suddenly coming to in a room they don't remember ever walking into and holding a knife. Later turn it up a notch, you can have a maid who is looking at her own hands with horror and intense guilt and if asked what's wrong she begins sobbing uncontrollably about how she didn't mean to do it and then when pressed (or the next room is investigated) you discover the body of her son. These people are slowly losing themselves to the aboleth and the players are no exception. So to save the townspeople (maybe another person among the residents is horrified about the murder of the child for a more personal reason, they can feel the impulse in their own mind and they begin begging others to kill them so as to save their kid from a similar fate) and themselves they must investigate the source of the whisperings and destroy it. When they eventually (hopefully) emerge victorious, I'd say play it more subtle. Don't have a bunch of obviously mind controlled folks. Have a few folks who seem perhaps just a little more absent minded than usual, maybe it's simple forgetfulness like when you go into the kitchen and forget why or maybe it's something more... sinister.

Also helplessness is another theme of horror games. Don't make it easy for the players to keep their gear or for them to stay together. Being one guy by himself with no weapons is way more terrifying than being a part of an armed group.

[minute]

Love the setup, but I would consider re-working it to be a lower-level adventure, Level 4 at the highest. Even the most disturbing and psychologically devious horror loses something when your heroes can take a knife in the back without sweating, and can cast stuff like Wall of Fire or Death Ward, to say nothing of Raise Dead. At low levels, the characters are extremely vulnerable.

The only new problem this creates is that your players are no longer capable of realistically fighting the Aboleth, to which I say... good? Horror adventures benefit, in my opinion, from an enemy too powerful and terrifying to be fought directly. You can still design an action climax, instead of fighting the Aboleth, can be fighting off the cultist slaves in order to reactivate the Six Seals which will imprison it, or to find the Flute of Wat'Yvr that will lull it to slumber. And during that fight, the Aboleth can send nightmare visions or something to disrupt them and help things feel extra nightmarish.

And if the players ignore your signposting and try to take the Aboleth on directly... they die. Or get enslaved. Or some of them die or get enslaved, leaving the others to flee raving and screaming into the night. It's a horror one-shot, is it really a problem if that's how it ends?

I'm definitely gonna ask the players if they're okay with not exactly "winning" in the traditional sense with this adventure. I mentioned one ending idea in the post, but you just gave me another one where after defeating the cultists in the "final battle" at the end, the last thing that they see is dozens of hulking, squid-like gray entities with gaping mouths and tentacle-like tails emerging from the lake. It'd be a very "fine, I'll do it myself" moment for the aboleths. But either way, I think you've definitely convinced me to not have the aboleth be the action climax at the end. Whether I keep it low level or not, the scariest monsters are the ones that you can't see, right?