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Megadungeons, Megacities, and Megamysteries

Megadungeons, megacities, and megamysteries are three campaign structures too straightforward to require complete articles, so we’ll package them together here at the end. A megadungeon is just a single dungeon made so unspeakably vast as to be able to sustain an entire campaign by itself, and a megacity is the same thing applied to urbancrawls. In both cases the only advice particularly needed is already included in the Art of Adventures. In the case of a megadungeon, just make sure you maintain enough variety in both opposition and layout that it doesn’t get dull.
In the case of a megacity, you’re using an urbancrawl to string together lots of encounters and adventures just like with a hexcrawl, and unlike with a hexcrawl there’s no particular process for moving from one location to another. Make sure you have enough content to last 13+ sessions, create a city with enough neighborhoods to contain that content without getting more than 3-4 adventures deep in one neighborhood, and that’s it. Creating that content is obviously not a small task, but it’s all encounter and adventure design, the campaign structure is only slightly more complex than a serial campaign.
A megamystery requires only slightly more elaboration. A mystery campaign is built on the same three clues principle as a mystery adventure, but rather than having the clues contained in a single adventure, some scenes have the clues to them spread throughout multiple adventures. The final scene of an introductory adventure contains three clues, each one pointing to the introductory scene of a completely different mystery adventure. These adventures, in turn, contain more clues to other adventures (some in their final scene, some in other scenes). So long as each mystery contains at least three clues each pointing to other mysteries, and each mystery has at least three clues point to it from other mysteries, it can be reasonably safely assumed that sooner or later players will find all of them. Just like with the standard three clues method from the adventure, if the players miss all three clues, new clues can be produced by having the villains advance their agenda in some way that leaves a new mystery behind, with this mystery containing clues to the start of whatever mystery the players have missed. If this happens often enough, the villains might advance their agenda to the point where they actually succeed in whatever dark ritual they have cooking up, thus prompting the boss fight with Cthulhu at an earlier (perhaps much earlier) level than was planned, which would probably end poorly for the players.
Players need to be prepared to take lots of notes for a full-on mystery campaign, both because this will help them catch clues and more importantly because it will help them keep track of the leads they’ve already found. A single mystery contains clues pointing to as many as three other mysteries, and if players manage to find all three, they could quite possibly forget one of them by the time they’ve solved the other two, especially if they discover even more mysteries while investigating those two, and follow up on those before going back to the one leftover from the first set.
The main question for a mystery campaign is how to structure the ending. You can always just have the climax mystery be sitting on top of a large tower of mysteries. The intro mystery has clues leading to mysteries A1, B1, and C1. A1 has clues leading to B1, C1, and A2. A2 has clues leading to A3, B2, and C2, and so on, just like with a mystery adventure. This is a perfectly reasonable way to build a mystery campaign if you don’t mind risking that a lot of your work will be missed and that at least some of it is virtually guaranteed to be missed. It’s unlikely, but entirely possible, that players will go from the intro to A1, to A2, to A3, to the climax, completely skipping over all six adventures in the B and C columns, over half of the work you’d done. It’s quite likely that players will get through at least one level of the tower without completing all three mysteries on that level (i.e. they’ll do A2, get a clue to C2, then from C2 find the clue to C3 and go up to the third level of the metaphorical tower without ever completing B2).
One way to solve this is by making the goal to solve every mystery, not just the topmost one. For example, rather than the goal simply being to find the head of the cult and stab him before he can summon Cthulhu, perhaps Cthulhu’s dreams influencing cultists and monsters and spawning cyclopean horrors to bring about his own awakening, which means there is no head of the snake to cut off (technically there is, but it’s Cthulhu, who is a high-level encounter if he’s beatable at all), so the players need to put a stop to each cult and star spawn and so on trying to awaken Cthulhu in various different ways. This issue lacks a final confrontation with any kind of central antagonist, but if that’s not something you and your group need for a satisfying conclusion, this method works perfectly well.
Another way of solving the problem is by having the mysteries be an interconnected web but the opposition more linear. For example, the one mystery is about discovering the location of Rlyeh and stopping the cultists from going there to wind up Cthulhu’s alarm clock, another is about preventing them from performing a mass ritual sacrifice when the stars align, a third is about stopping them from recovering the Necronomicon and reading from it the magic spell that will awaken him (I’m aware that I’m mangling Mythos lore a bit here, but translating Lovecraft faithfully into D&D is not the point of this example), but then the actual specific cult lieutenant attempting to complete these rituals happens in the same order no matter what order the players discover and investigate the mysteries. The first one the players investigate is always a goblin cult, the second one is always a vampire and his spawn, the third one is always a mind flayer and his thralls. This has the disadvantage that you can’t make any of your clues have to do with the antagonist of a mystery because the antagonist is undefined until the players begin to investigate. You can’t have the players learn about the cult’s trip to Rlyeh by reading correspondence where they talk about how the mind flayer is off to Rlyeh because maybe the mind flayer is actually the one chasing the Necronomicon. The advantage, however, is that the final confrontation will always be with the primary villain.
You could also have the final confrontation trigger after all mysteries are solved and be some other sort of adventure entirely. Maybe each cult lieutenant holds one fragment of a key to Rlyeh, and the last adventure is a dungeon crawl through the primordial city. This method can be done in stages, so that one web of mysteries, after being entirely cleared, leads to a wilderness journey, which in turn leads to a new intro mystery that leads the party into a new web, thus providing the occasional non-mystery adventure for variety.
Finally, you can always just ask your players to play along. “Yes, you’ve found a lead on the head of the cult, but you also have three other leads you haven’t followed up on, and you’re going to need to chase them all down eventually, so how about we leave the final boss for the end?” This has the disadvantage that it’s a really out of game reason to put off the head of the cult, but if the other methods all require drastic overhauls to your campaign that you think will do more harm than good, there’s nothing particularly wrong with using this option.