As anyone who watched Star Trek: The Next Generation will attest, the most popular pastime in the 24th century (provided sinister aliens weren't involved) was something called "Tri-Dimensional Chess". Every single table in ten-forward seemed to be equipped with a Tri-Dimensional Chess board. It used to fascinate me in a mild sort of a way. How on earth could such a game be played? The rules were never made clear, but it seemed to involve a number of games of chess taking place simultaneously and in parallel, so that what happened on one board would impact the others and vice versa.
For some reason I was thinking about Tri-Dimensional Chess on the way to work today and I started to ponder what the D&D equivalent would be. This would be a form of D&D taking place across a number of different dimensions simultaneously, with constant slippage between the different planes. I call it The Tri-Dimensional Planebox Megacrawlathon SandhexamagigTM.
Picture the scene. Bob, the DM, slaving away over hexmaps and graph paper, planning plot seeds and adventure locations and prominent NPCs - doing all the things you do to prep for a new campaign. Except, what's this? That isn't one hexmap that Bob is creating. It's 3, or 7, or 9. Wow - that's a lot of extra effort to put in; is he going to be running a game for a different group every night of the week? But in that case, why not just use one hexmap and recycle it for each group?
That's because he's only DMing for one group, but the game they are playing is the Tri-Dimensional Planebox Megacrawlathon SandhexamagigTM version of D&D.
Look closely. There is method to Bob's madness. He isn't plotting out 3, or 7, or 9 different hexmaps. He is plotting out 3, or 7, or 9 variants on the same hexmap. Where in hex 7D on map 1 there is "Orc Lair", on hex 7D on map 6 there is "Laser Ape Lair", and on hex 7D on map 4 there is "Octopod Mutant Lair". Where in hex 8E on map 1 there is "Rothbury (town)", on map 6 there is "Kalakilakik (mothman town)" and on map 4 there is "Howick (town - Deep Ones)".
In Tri-Dimenstional Planebox Megacrawlathon SandhexamagigTM, you see, each campaign takes place across several planes of existence, all of which are different and yet the same: reflections of each other, refractions of the same light. One is high fantasy, one is Arabian Nights, one is Lovecraft, one is Space Marines, one is Sword & Sorcery, one is Everything is Dolphins.... Occasionally, there is seepage in between: the PCs may slip through portals to the other dimensions and back, and what they find on each plane will remind them of their home dimension and yet be subtly - or hugely - different. Locations, magic items, NPCs, geography, will all be familiar, and yet at the same time not. And perhaps, over time, their actions in one plane will come to warp the others: if an NPC on the first plane dies, perhaps the other versions of that NPC all fall foul of mysterious maladies, accidents or mishaps.
And of course, roaming all of the other planes of the Tri-Dimenstional Planebox Megacrawlathon SandhexamagigTM are people who look horribly familiar still, because they are variants of the PCs themselves, with agendas of their own....