The "multiverse" existed as far back as 1e's Deities & Demigods. (If not the 1e DMG.) Way back in 1980.
No. I disagree with this assertion, and would require evidence for it. While there may have been "other planes," there was certainly no overriding consensus in the 1970s and early 1980s that all homebrew, 3PP, and other campaigns existed within the greater D&D multiverse. I mean, maybe I didn't hang out with the cool kids, but I certainly wasn't aware of this.
I was wrong here.
Discussion on the multiverse takes place in Appendix IV of the AD&D Player's Handbook. Page 120. So a couple years earlier than I assumed.
lowkey13 is right about this.
The "multiverse" has its canonical beginnings in the 1st ed MotP. The rest of this post will elaborate.
Page 120 of the PHB (Appendix IV) says this:
The Prime Material Plane (or Physical Plane) houses the universe and all of its parallels. It is the plane of Terra, and your campaign, in all likelihood.
Pages 57-58 of the DMG (under the heading "Travel in the Known Planes of Existence") says:
The Known Planes of Existence, as depicted in
APPENDIX IV of the
PLAYERS HANDBOOK, offer nearly endless possibilities for ADBD play, although some of these new realms will no longer be fantasy as found in swords & sorcery or myth but verge an that of science fiction, horror, or just about anything else desired. How so? The known planes are a part of the "multiverse". In the Prime Material Plane are countless suns, planets, galaxies, universes. So too there are endless parallel worlds. What then of the Outer Planes? Certainly, they can be differently populated if not substantially different in form. . . .
For those of you who haven't really thought about it, the so-called planes are your ticket to creativity, and I mean that with a capital C!
Everything can be absolutely different, save for those common denominators necessary to the existence of the player characters coming to the plane. Movement and scale can be different; so can combat and morale. Creatures can have more or different attributes. As long as the player characters can somehow relate to it all, then it will work. This is not to say that you are expected to actually make each and every plane a totally new experience - an impossibly tall order. It does mean that you can put your imagination to work on devising a single extraordinary plane. For the rest, simply use
AD&D with minor quirks, petty differences, and so forth. If your players wish to spend most of their time visiting other planes (and this could come to pass after a year or more of play) then you will be hard pressed unless you rely upon other game systems to fill the gaps. Herein I have recommended that
BOOT HILL and
GAMMA WORLD be used in campaigns. There is also
METAMORPHOSIS ALPHA,
TRACTICS, and all sorts of other offerings which can be converted to man-for-man role-playing scenarios. . . .
uppose that you decide that there is a breathable atmosphere which extends from the earth to the moon, and that any winged steed capable of flying fast and far can carry its rider to that orb. Furthermore, once beyond the normal limits of earth's atmosphere, gravity and resistance are such that speed increases dramatically, and the whole journey will take but a few days. You must then decide what will be encountered during the course of the trip - perhaps a few new creatures in addition to the standard ones which you deem likely to be between earth and moon.
Then comes what conditions will be like upon Luna, and what will be found there, why, and so on. Perhaps here is where you place the gateways to yet other worlds. In short, you devise the whole schema just as you did the campaign, beginning from the dungeon and environs outward into the broad world - in this case the universe, and then the multiverse. You need do no more than your participants desire . . .[/indent]
This is not a canonical "multiverse" in which everything exists and every campaign world is to be found. It's a worldbuilding conceit to allow the GM to include variety, to run sci-fi crossovers, etc. A GM may take the view that some other GM's campaign world is part of his/her campaign's multiverse, but there is no explanation that this will be the default. As p 111 of the DMG says,
Transferring player characters from other campaigns to yours is appreciated by the participants coming into the milieu, as they have probably spent a good deal of time and effort with their characters, and a certain identification and fondness will have been generated. You can allow such integration if the existing player character is not too strong (or too weak) for your campaign and otherwise fits your milieu with respect to race and class. The arsenal of magic items the character has will have to be examined carefully, and it is most likely that some will have to be rejected.
Again, we see the "multiverse" as a gameplay conceit - parallel prime material planes create a homeland from which these transferees have travelled to the new milieu, and their loss of magic items can be explained as a consequence of that travel: as p 118 says, "Simply inform the person that he or she must have left the item in his or her former area, as it is not around in yours!"
The difference that MotP makes is this: it is the first presentation that assumes that all the DDG deities all exist, concurrently, in the various planes of existence. DDG itself assumed that a GM would choose which gods are part of his/her campaign world - so that the book was a catalogue of choices for a GM to make, not a catalogue of the inhabitants of the planes. MotP changes this fundamental assumption, and is the bedrock on which later "multiversal" endeavours like Spelljammer and Planescape are built.
That is not to deny that there may have been some players who, prior to MotP, imagined all the gods in DDG as co-existing; or who imagined that every D&D setting, from the published ones like GH and Boot Hill (a setting to which Murlynd had notoriously travelled) to the setting that they once made up for a one-shot on a lazy Sunday afternoon, all co-existed as parallel prime material planes. But that view was just one possibility, and enjoyed no canonical status.
the "D&D multiverse" has its own continuity. The multiverse has a backstory.There are assumptions and commonalities.
While you can run a D&D game in your own world that is part of the multiverse, or run a D&D game in your own world that is completely and totally different. This this is like how you can take the Numenera rules and use them to run a game not set on Numenera. Or a Marvel Heroic Roleplaying game that isn't set in the Marvel Universe.
Nonsense.
That MHRP will take place in the Marvel Universe is a core conceit of MHRP. It is there from the opening pages of the book.
Nowhere does D&D assume that my D&D game happens in some component of a common "multiverse"; there is no assumption, by default,of some sort of continuity between campaigns that I run.
The 4e Rules Compendium, for instance, says this (pp 7-8, 54):
The world of the DUNGEONS & DRAGONS game is a place of magic and monsters, of brave warriors and spectacular adventures. It begins with a basis of medieval
fantasy and then adds the creatures, places, and powers that make the DUNGEONS & DRAGONS world unique.
The world of the DUNGEONS & DRAGONS game is ancient, built upon and beneath the ruins of past empires, leaving the landscape dotted with places of adventure and mystery. Legends and artifacts of past empires still survive - as do terrible menaces. Although minor realms exist, they are widely scattered points of light in the surrounding darkness that shrouds the world. Monsters and supernatural creatures prowl the dark spaces. Some are threats, others are willing to aid the adventurers, and many fall into both camps and might react differently depending on how the adventurers approach them.
Magic is everywhere. People believe in and accept the power that magic provides. True masters of magic, however, are rare. At some point, all adventurers rely on magic. Wizards and warlocks draw power from the fabric of the universe. Clerics and paladins call down the wrath of their gods to sear their foes with divine radiance, or they invoke their gods’ mercy to heal wounds. Fighters and rogues don’t use magical powers, but their expertise with magic weapons makes them masters of the battlefield. At the highest levels of play, even nonmagical adventurers perform deeds that no mortal could dream of doing without magic. . . .
The preceding sections sum up the basics of what the game assumes about the DUNGEONS & DRAGONS world. Within those general parameters, though, there’s a lot of room for the DM to fill in the details. Each published campaign setting describes a different world that adheres to some of those core assumptions, alters others, and then builds a world around them. Any DM can do the same to create a unique, personalized world.
As with Gygax's DMG, we see the conceits of the gameworld presented as ideas for worldbuilding by a referee, not as accounts of the nature of a common D&D multiverse.