Running a Time Loop

My long-running 5e game is finally heading towards a close and I've decided to try and fit in as many of my DM-bucket list things before it reaches the finale. One of these is a time loop, something that's been on and off my mind since I read an Angry GM post about this as a teenager in 2018 or so. Then, a couple of months ago, I began to prepare an adventure site to wrap up one of the major threads in this campaign, and realised it could be an opportunity to tick this type of adventure off.

The setup

The main design goals for this are:

Puzzle-type challenges that reward a second attempt with more information.
Evoke a movie time loop feel, where NPCs behave like clockwork.
Keeping it fairly tight so that each loop is pretty much a one shot.

There's another dimension though: the adventure site is the family home of one of the PCs and their parents are important nobles in the setting, who practise shadow magic and arcane experimentation. There is an angle here that could be used for the time loop, where these experiments have somehow gotten out of hand and caused some arcane catastrophe. I thought it would be more interesting, and maybe more fun, for the time loop to be an illusion. The parents know there is hidden knowledge relevant to their goals in the manor, but that it is dangerous and inaccessible. They also have memory wiping powers and a track record of using family members as lab rats. So my thought is that they've set up this situation in order to trial some new memory techniques on the party, while using them to solve the puzzles of the manor and find the secret knowledge.

This is quite mean and might make the whole thing harder to pull off, but leads to two very fun outcomes: either a big reveal when they swoop in at the end, or the party discover the ruse and are able to exploit it for their own benefit. It also raises the stakes, as time is actually passing with each repeat and the PC's various opponents will be making progress on their own plans.

The Shadowmaster Manor

The Shadowmasters are a matriarchal family of half-orc shadow sorcerers. They are served by the Shadowguard, a lineage of goliaths who act as bodyguards. The Shadowguard were once cultists guarding a portal to the plane of water, and the Shadowmasters their prisoners who would be sacrificed to summon a Nightwalker when the portal needed defending. When one Shadowmaster and her guard fled, they took the key to this portal with them, and it is hidden within the Shadowguard's cottage on the Shadowmaster estate to this day. This information is unknown to the Shadowmasters and was only somewhat known by the Shadowguard, who have (player backstory) been murdered in the past few years by other factions in the world. The players and their enemies are searching for these portal keys though, and know one is connected somehow to the Shadowmaster family.

Wedge shaped rails

To keep things tight, the plan is to have the players on a 4 hour clock (24 exploration turns) and push them pretty hard towards the Shadowguard cottage as time goes on. I'm planning on achieving this using a fire in the manor that spreads from the 2nd to 3rd hour, and acid rain in the grounds starting at hour 3. These will be achieved in game through the mirage arcane spell that the current matriarch has, to minimise clean-up on repeats and allow for shenanigans if they realise. The other main thing at the Shadowguard cottage is the guardian of the inner sanctum, whose job is to be a hard gateway. Only Shadowguard are allowed into the inner sanctum and they're all dead, so the players will need to work out a way around this to progress on the loop. Hopefully the first couple of times they trigger the guardian, and this can act as a cap on each run. It will deal failed death saves on a hit, or something equally punishing, to provide a quick ending to events and allow the next loop to be set up. Through this combination I hope to give a clear structure to the time loop, approximately:

Explore the manor and grounds, finding useful information for once they get to the cottage.
Get pushed towards the cottage by worsening environmental effects.
Apply what they've learnt to try and get past the Shadowguardian. On each repeat, they can address targets in the manor quicker and expending fewer resources.

Once they do get past the guardian, either through somehow getting a Shadowguard (there's a like robot version of the most recently killed one somewhere in the manor, as well as rites to make someone a Shadowguard) or through going crazy in combat, there's a small dungeon where they can eventually find the key and everything can come to a head.

Net crawl?

In order to achieve the clockwork NPC effect, I think there are two main options. The first is to keep very detailed notes of how everything plays out and use these to make it seem the same every time. The other is to actually prep each location at each time step in detail. The first is obviously easier in terms of prep but the second would I think lead to more rewarding play, at least for my DM abilities. It's the perennial issue of running heists in these sorts of games and this is basically a time-heist. To make it a little simpler I'm thinking of treating the whole thing as a sort of 3D pointcrawl. There'll be a small number of fixed locations with paths between them, and they'll have a key for each time step where they are typically accessible. Another benefit of the fire and acid rain is reducing the number of keys I have to make. In most cases, I think I'll keep these very simple and can use a line or two of boxed text to get the movie time-loop feel. There's maybe also a quantum approach that could be used, where the first interaction with each NPC is fixed or something, but I think this could maybe overcomplicate things.

I've got nine locations, with at least six paths, and 24 time steps. That's a lot of keys (248 currently lol). I think my approach will be to key out each location in a neutral state, work out the paths of important NPCs throughout the loop, and then do boxed text based on the combinations of these. For points without NPCs at that moment in time, I could maybe use some big table to generate ideas to create some evocative and memorable little tidbit that triggers regardless. The landmark, hidden, secret approach will be very useful throughout this and I think should be the approach to most encounters here, as it gives a lot of purpose to the loop. Once you've worked out the extra layers for an encounter, it should be quicker and easier to overcome. Here are a couple, one for the whole thing and one for the inner sanctum.

Events are repeating.

The fire and rain are an illusion.
- The Shadowmasters are faking a time loop to get the union to find the key.

The Shadowguard inner sanctum is protected by a powerful creature.

The guardian is not summoned if a Shadowguard enters first.
- There are secret rites to make someone a Shadowguard.

The plan

I don't know how long it'll be irl until the PCs pay a visit to the Shadowmasters but it's not far down their agenda at the moment so I probably need to move quickly. I'm also going to be very busy with real life things in September so won't be able to do much prep. I might also not be able to run games as often though, so it could even out. Regardless, the sooner the better with all of this.

Sketch the pointcrawl.
Key each room.
List the relevant NPCs and their usual pathways.
Make a big themes table.
Add boxed text for each time increment to the keys.