How to prevent planar travel to your world.

[Bendris Noulg]

As for Gate, I don't believe you need any sort of connection between the planes in question. Nowhere in the PH or Manual of the Planes does it mention Gate not working when trying to create a portal to independent demi-planes, etc., even though they do take the time out to talk about how specific PH spells work in a planar setting (in MoTP).

Actually, what astralpwka just posted...

What I do is have it as a separate cosmology, even though it has similar planes. Then, instead of calling them planes, call them dimensions. That's what I'm doing with my next campaign site.

...Is close to what I'm talking about (and definately fits the criteria as well). Essentially, a plane can be independant of a in-total cosmology, free-floating in some kind of morass of "nowhere", requiring a specific gate, key, artifact, or Epic Level Spell to reach, access, and/or escape (in this case, I would personally recommend an artifact in the form of a temple alter requiring a number of keys that must be found in order to open the gate, but that's because I'm planning something similar, so YMMV ).

 

[Lord Pendragon]

Unless you are drawing from Spelljammer rules or some such, I don't see exactly what you mean. Perhaps I'm just not looking at it the right way. A cosmology isn't just the Prime Material and any planes connected to it. It's a term for all the planes in existence. I.e. every free-floating plane in Greyhawk, in spite of what portals it may have and where they lead, is a part of the Greyhawk cosmology. Likewise FR or a homebrew.

As far as I know, only Spelljammer plays with the idea that Greyhawk and the Forgotten Realms both exist simultaneously. In a (standard) campaign based in either setting, the other cosmology does not exist and nothing can exist outside of the one that does.

So to say that something can exist "outside of the cosmology," to me, is like saying you could have your demi-plane in somebody else's game. It's an odd sort of meta-gaming.

AFAIK, anyway.

 

[Saeviomagy]

Unless the players can already cast planeshift, you could always just limit high-level magic. The BBEG sends them to a plane of extremely limited magic (say a maximum of 5th level spells).

Simply put - every spellcaster is now limited to a spell level equal to half (rounded up) of his original maximum spell level. That'd bump the lowest level manifestation of plane shift to 17th level minimum. If you want them to have it earlier, they need to find an item which can cast the spell, or a natural portal.

 

[Bendris Noulg]

AFAIK, anyway.

Understood...

However, each cosmology is (supposedly) connected via the Plane of Shadow (see illustration on page 62, which includes the Great Wheel and the FR Cosmological Tree). Also, you'll notice the inclusion of a "lone plane" in the Myriad Plane cosmology on page 215. So the precidents are there, just a matter of expanding on them as needed.

 

[Psychotic Jim]

Here's a few far-out ideas if you are really desperate:

-kill them. They arise on another plane as petitioners (as described in the Manual of the Planes) in their afterlives . Being petitioners, they are bound to this plane and cannot leave through any normal means. Somehow, unlike most petitioners, they have retained their former identitiies and class abililties. The BBEG should problably destroy their old bodies back on the Prime though, so a friendly NPC can't simply cast Ressurrection or Raise Dead on their corpses.

-have the BBEG transform them into demonic fiends and banish them. I seem to recall somewhere that demons cannot enter the Prime Material Plane unless specifically summoned by mortal spellcasters.

- suck them into a rift in time, perhaps to a time when their homeworld was not formed yet, had already been destroyed, or had become so alien that the PCs do not recognize it. Most spellcasters can't deal with time-travel. This would work best if you made the players think they just shifted to another plane and not realize a time jump occurred.