For level 18, my part in my Ravnica-based campaign have been assembling the shards of the Golgothian Sylex, an ancient and mysterious doomsday weapon that destroyed an entire continent on the plane of Dominaria, and is the tool they need to destroy the Phyrexians' "Cosmic Key," which Elesh Norn and company are going to try to use to redirect various portal/wormholes the Mind Flayers created linking planes of the Magic the Gathering Universe with those of D&D. Essentially, the Phyrexians want to use these passages to quickly get their forces to planes they'd normally be unable to visit because of the Post-Mending barriers to planar travel for non-planeswalkers (I'll note that I came up with this plot before any of the March of the Machine stuff happened in MtG canon).
Anyway, this has been an opportunity for the party to visit other Magic planes. So far, they've gone to Theros, Innistrad, Arcavios, Ikoria, and are now on Eldraine. The last of the six shards for them to find is on New Capenna.
And I still haven't totally finalized what that adventure will be like.
Part of the premise here has been to do something a little different on each plane. Innistrad had the party forcibly split up by the Mists (Innistrad is, in my campaign, linked to the Shadowfell and thus there's a sort of quasi-Domain of Dread there). Arcavios was a series of relatively combat encounters with unconventional win conditions for each college. Theros was a journey down into the Underworld and ended with a race out of a labyrinth within Erebos' palace. Ikoria was a hunt for the Tarrasque. In Eldraine, the party has been transformed into children and must do a series of challenges in tier 1 levels (starting at level 1 and leveling up after each, and then getting restored to 18 and adulthood when finished).
So: the premise for New Capenna is that the shard was entrusted to the Maestros, who keep it as a relic of Old Capenna (while it wasn't from there, it was instrumental in fighting the old Phyrexians, who likely still control the surface far below).
On a previous planeswalk, the party's Goblin Bard came to New Capenna and Xander, the leader of the Maestros, sought to recruit him to work as an assassin for them. I do think that there's a "speed run" cheat code here - if the party contacts Xander and entertains an offer, he'll allow the party to take the shard if the bard pledges 999 years of service and agrees to be transformed into a demon in order to carry out his duties (the Bard is already a chaotic evil member of the Cult of Rakdos and both a serial killer and cannibal, so this might not be completely outside the realm of possibility - though his greatest virtue is his loyalty to the party). This service would take place post-campaign, so there's no real gameplay cost to it. I think Xander could be negotiated down to 101 years, but if the offer is either refused or not entertained at all, I need to figure out how to do this adventure.
In theory, the Museum of Old Capenna, where the shard is on display, could be run like a somewhat conventional dungeon, though given that the party is level 18, I'd need to pull some really crazy BS to prevent the party from steamrolling it.
The alternative, and the one that I'd be really curious to try, is borrowing the structure of Blades in the Dark. Specifically, it would be up for the players to decide what kind of barriers to their goal are as they choose to use various skills and abilities.
I've put a little thought into this: First off, because skills (especially at high levels) in D&D can be enormous bonuses, we'd have a system where re-using a skill would raise the DC. You might have a +13 to Arcana (which our Aritficer does - and if they use a tool that becomes +20) but eventually, the DC for using it over and over again will become prohibitive or at least risky.
We'd have clocks that need to be filled in order to progress through the heist: I imagine there's a Casing clock, an Infiltration clock, and then a secret, third clock in which the party realizes that the Shard on display in the museum is actually a fake, and the real one is down in the museum archives.
Each of these would then be put up against a Security clock. I think the penalty for failure is to have to do a High-difficulty combat encounter (which at level 18 is quite a lot). The heist has to go off in one night (otherwise the Maestros will move the shard) so I think we can put some pressure on the party having to get through multiple really tough encounters if they fail.
I think I then just need to build a map for each of the fail-state encounters (the last would probably have a fight against Xander, who I'd give the stats of Grazz't, along with several henchmen - probably Bandit Crime Lords among others).
The rising DC remains through the entire night - probably starting off with an easy 10 but going up by 5 each time it's used, and everyone's got to do their own thing (we'd basically go in initiative, even if we're doing something that takes place over the course of hours). I might make Tools also raise the DC with repeated use (the Artificer is primarily a glazier).
There are six regular players, so I think a progress clock would be in twelve segments. Failure clocks, then, I think are going to start off lenient (maybe also 12) but go down by two each time, so that the second phase (once they're actually breaking into the museum) is 10 and then searching the archives would be 8. This way, the tension rises and failure gets more likely as things go on.
Cool, I think this can work: just need to make three maps and encounters.
We'll see how it goes!