Hi all — I’m stuck for inspiration for my next session and could use some fresh perspectives.

In my campaign, a castle has appeared overnight out in the far wilds. The design is clearly ancient — far, far older than the region (which has only been inhabited for about 100 years). I’ve got a few possible directions but wanted to hear what others might suggest or build onto.

Here are my ideas so far:

1) The Land Mimic

The castle isn’t a castle at all — it’s a colossal mimic, a sort of “land mimic” that imitates terrain features or buildings.
Because of its massive size it’s slow, more like a giant Venus flytrap: it waits for prey to walk inside.
Inside, the party slowly gets ensnared by charming effects, shifting rooms, and digestive traps.
(It would need extra abilities so it’s more than just a scaled-up mimic.)

2) The Philadelphia Experiment / USS Eldridge Scenario

The castle is real — it disappeared centuries ago and has just popped back into existence unaged.
No people remain, but there may be monsters that came with it… or gnarly side-effects like:

• people fused with walls

• time distortions

• rooms that phase in/out

• a collapsing structure pulling the PCs into whatever dimension it was trapped in

The castle becomes more unstable as the party explores, forcing a race to escape.

3) The “Metallic Rose” Ghost-Castle

Inspired by Metropolis / Metallic Rose:
A grieving noblewoman spirited her entire castle away into another plane. Now it has crashed back into the world.
She “haunts” it — not a standard ghost, but an intelligence woven into the structure itself.
She fixates on one or two PCs, trying to shape them into the image of her lost lover, while isolating or tempting the rest.
If the PCs escape, one of them might continue to dream of the castle… or fear they never actually left it.

(I’m considering letting players not in a scene temporarily play illusions/dopplegangers of themselves to make it extra eerie.)

I’d love any thoughts, twists, or additional ideas for ‘castle that suddenly appears’ scenarios.

Thanks in advance!

I prefer of your three, the USS Philadelphia story.  But I do think some people/npcs are there.  But the people/NPCs can speak, be heard, but do not see the PCs.  Once the party thinks that is what is going on, let an NPC walk through them.

I personally like the idea of walking through a castle, walking into a kitchen, spending some time, and then trying to take the door to leave, only to discover that it leads into a different room. "wait, that's odd" players will think. "wasn't this bedroom meant to be the hallway? lemme go back into the kitchen, see what happens." then they get D I G E S T E D.

number one, definitely a fave if you like to mess with your players heads.

Oh my god! The Philadelphia Experiment! There's a name I've not heard in a long time! 

So here's my idea, for what it's worth:   This castle was here a long time ago, but the wizard who lived there was conducting experiments in Chronurgy Magic (Explorer's Guide to Wildemount) and had an accident. The castle blinked out of existence and then reappeared in the same spot in the distant future (now). Of course, there has to be a sign by the front gate identifying it as "Eldridge Castle". Venturing inside, the party will find people fused in or through solid surfaces, and doorways that lead to random rooms, making mapping the castle and finding their way out nearly impossible. While they are exploring the castle, and getting lost along the way, require them to roll a Constitution saving throw every few minutes. Start with a low DC, like 5, but gradually raise the DC. Each time someone fails the save they gain one level of Exhaustion. This is just to make them hurry, so they're not here all day and won't want to take a rest here. Eventually the party will locate the wizard's lab, where the experiment was being conducted. As soon as they open the door they see ghost-like things flying around the large laboratory (at least 60' square). These are the spirits of the people who died in the castle, and have been driven quite insane over time. They may also notice that every object in the room (wooden tables, metal candelabras, shelves of books, tapestries, everything) appear to be decaying at an advanced rate. But at the other end of the room a rotting skeleton sits in a chair, it's wizard robes already turned to dust on the floor around it, and an ornate silver model of a ship sits on the table in front of it. A silver necklace containing a large gemstone still hangs around its neck, and this item does not appear to be decaying. (Clearly it's a powerful magic item that they will want). Once they step inside the room, at the start of each turn each character must make a Constitution save, DC 25. On a success that character ages 5 years. On a failure that character ages 25 years. But simply running in, grabbing the necklace, and running out, won't be so easy, because all those ghostly creature will be attacking them. Also, once they enter, the door will slam shut behind them and lock. So even if they defeat the ghost creatures and grab the necklace, they still have to either break down the door or pick the lock to get out (DC 20 either way). So if the party is all elves, the aging might be a minor inconvenience. But if they're most other races, within a few round they will be crumbling elderly versions of their former selves, with decreased ability scores to match, which will make breaking down the door a lot harder!

But even if they do manage to get in, grab the necklace, and get out, they're not done. They notice the entire castle is starting to glow and shimmer, which should be ominous enough for them to want to GTFO right away. But doorways don't always go where they should! So the simple mechanic here would be to have whoever is leading the group roll Intelligence saving throws to see if they can remember the sequence of the path they followed on their way in. Let's say it's just a DC 12 or maybe 15. If they succeed on 3 saves before they fail 3, they find their way outside just in time to see the castle vanish. If they fail 3 saves before 3 successes, they vanish with the castle into some distant future version of that same location. So of course you want them to succeed.

So assuming they get out with the necklace, they're all gonna yell, "What is it!" Well, we'll have to make something up. I like magic items that cost something to use. So first someone will have to wear it and attune to it. So they get a couple little things, like their movement speed increases by 10 per round and they can make Initiative rolls with advantage. But the necklace will also allow them to use Action Surge to take one additional full action on their turn. BUT! Doing so causes them to take 1d6 Necrotic damage and they have to make a Constitution saving throw or instantly age 10 years older. The DC starts at 10 for the first use each day, and increases by 5 for each additional use per day. 

Of course, much of the party is probably elderly now. So to fix that you can just handwave a rule that says one casting of Greater Restoration will remove 5 years worth of magical aging, and the Wish spell instantly returns one person to normal. Oh, and I also like throwing in inside jokes or obscure references into a dungeon. So when they're in the wizard's chamber for the final fight, if they examine the silver ship model they will see it's a strange sleek metallic ship of unknown design, with no sails, and the number 173 painted on the side of the hull.

So that's what popped into the ole' noodle. Sorry if I rambled. 

Its a good ramble. TBH I'm still not sure which to do, I like them all...I'll probably end up planning all three just to run at some point.

Quote from AnzioFaro >>

Of course, there has to be a sign by the front gate identifying it as "Eldridge Castle".

This is absolutely worth it if one of the players forms a warlock pact with the ghost of the wizard, so they can cast Eldridge Blast