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General Properties of Mystaraspace(a possible Planescape - Spelljammer - Mystara connection)
by Marco Dalmonte
Mystaraspace Crystal Sphere
Mystaraspace lies on the Prime Plane like the rest of the Crystal Spheres. However, the Mystaran crystal sphere takes the form of an energy field that surrounds Mystaraspace. On passing through the field one may end up in the Flow of Spelljammer, or in the old D&D Universe (i.e the Milky Way, also called the Mysterious Path). In the Flow, looking back at the Mystaran crystal sphere, it appears perfectly normal. In the Milky Way, looking back at Mystaraspace, the system is surrounded by an energy field. The idea is that the Mystara Multiverse and the AD&D Multiverse are two different "dimensions" touching in Mystaraspace.
Now, how can one determine if his Voidship will end up in the Phlogiston or in the Milky Way? As a role of thumb, a spelljamming engine will be always drawn to the Flow when exiting Mystaraspace, while any other type of motive force will simply take the ship into the Milky Way. The only way to avoid this on both parts is to open a Gate (or similar portal -Arcanes can manufacture such things) to the side they cannot reach. But here's the trick: if you don't know there is another way out of Mystaraspace, you'll never try it! (Spelljammer Portal locators will find only portals to the Flow.) In other words: if you don't know there's a hidden room in your house, you won't ever look for it, unless you stumble upon somebody who knows of the room...
So in essence, Mystara lies at a crossover point between two (and maybe more -see Nightmare Dimension) Universes or Dimensions (AD&D Planescape calls them Prime Planes, although I'd object this restrictive definition), like a sort of junction point.
The Immortal Council of Intrusions takes a far more active role than is probably implied in the Codex of Immortals. They act as "ambassadors" to all the extra-Mystaran Planes and Powers. Also, in an effort to maintain Mystara's purity (I recall that in the old Gold Immortals book it says something about how Mystara is the one spot in the Prime Plane where the 5 spheres are in perfect balance, and this also explains why so many Immortals have arisen from Mystara) the Council stops most summonings from the AD&D Outer Planes (no tanar'ri on Mystara planet). Since Mystara is at the junction between two dimensions, one could theoretically summon also fiends from the AD&D planes with the appropriate formulae (different from those used to summon D&D Fiends or the demons living on Demonworld -another world in the Astral), so it's logical Immortals try to prevent this. This obviously means there is an adversarial situation between AD&D Gods and D&D Immortals, with the latter trying to prevent the former meddling with Mystaraspace and its dimension.
Following are two possible ideas about a D&D-AD&D Multiverse connection.
D&D vs AD&D
D&D is a universe of its own, with planets, voidspace and stars. AD&D is another universe with Crystal Spheres and Phlogiston. In D&D universes the Outer Planes are located sparsely in all directions of the Astral, to which the Prime is somehow linked. However, since the first doesn't mirror the second, then it's a hard guess to understand where you could pop up if you changed plane in different places. In AD&D there is the Great Ring of Planescape where all Outer Planes have their ordered place. The common point between D&D and AD&D is Mystaraspace, the junction space.
This way, the D&D Outer Planes are in the Milky Way Multiverse, while the AD&D outer planes and such are still in the Flow Multiverse. So the Immortals live in the D&D Multiverse but can access the AD&D one as well: however, their home plane lies in another multiverse and they are subject to different rules and hierarchies than AD&D Deities.
D&D In AD&D
The old D&D Outer Planes are nothing more than AD&D demiplanes located in the astral and ethereal planes and clustered around Mystara. By "clustered" it seems to me that space in the Astral and Ethereal planes corresponds to space in the Prime - in other words, by "clustered around Mystara" I mean that if you travel from Mystara to the Astral or Ethereal you will be near all those demiplanes. Or to put it another way, colour pools around those Demiplanes lead to Prime Mystara. The AD&D Great Ring exists as well, beyond the Astral Plane, and all Mystaran Immortals' demiplanes are a sort of rivals to the Great Ring.
In AD&D Planescape there are multiple Prime Planes. However, this fact doesn't alter the other planes, so the "Milky Way" and "Spelljammer" may well be different Primes, but they would be just Universes, not Multiverses - they don't affect the Outer Planes at all. This is all biased in favour of squeezing Mystara into existing Planescape stuff, so that the Planescape DM can use it without having to change his campaign setting!)
As you can see, dear reader, you have just to choose which one you want to use in your campaign (again, DM's call applies).
Mystaraspace and Spelljammers
Talking about Mystaraspace and air envelopes, it seems that Spelljammers are at risk upon entering Mystaraspace. In fact, in this Crystal Sphere gravity and air envelopes don't function the normal (SJR) way (see the Mystara-Spelljammer Conversion Guide), so spelljammers really risk suffocating upon entering Mystara, or risk an internal mess (think about some ships like Tripods which are built exploiting multiple gravity planes). What I am proposing here is to use the rules described in Champions of Mystara, making this space/Crystal Sphere even stranger and unique (it is logical even for Spelljammer rules that each sphere has its own physical and magical rules). However, in order to "help" a bit the Spelljammers entering Mspace, I would add this little rule: every ship powered by a Spelljammer is able to retain an air envelope inside Mystaraspace. This doesn't last long however, but only 1 day per magnitude of the Spelljamming helm powering the ship (minimum 1 day, maximum 10 days). This way, it gives the SJR ships a bit of an advantage and also some time to understand what's happening and to find countermeasures before it's too late. This could also explain why there is so little traffic of Spelljammers in Mystaraspace, but some do come nonetheless.