World Portals

World Portal

9th level Magic User Spell
Range: 20'
Duration: 1 turn
Effect: 1 portal (up to 10' x 10')

This spell requires a specially created archway at least 6' high and 4' wide, up to 10' x 10', usually of precious metals and finely wrought (5000gp cost). This has a 95% chance of surviving the casting for reuse. When cast, an interdimensional portal is created within the archway that leads to another world on the Material Plane (not to be confused with other planes of existence). 

Similar to a teleport spell, the chance of success is based on how familiar the caster is with the destination:

• Known only to caster by name - 30%
• Caster has read a brief description - 50%
• Caster has detailed description, or frequent conversation with a native - 70%
• Caster has visited briefly (1 day or less) - 80%
• Caster has spent more than a day - 90%
• Caster is a native of the destination - 100%

Failure means that the caster successfully opens a portal to the wrong world. Unlike Teleportation there is no chance of either materialising in solid earth or high up in the air, but finding oneself on the wrong world is problematic enough. The exact location also depends on the caster's knowledge, but generally if they specify a particular place as well the world they want to reach, then if the caster gets the right world, they will reach that location. If a wide area is described, the portal may open in any part of that area, as long as it is relatively safe - once again DM discretion applies. If no location is specified, the portal may open anywhere in the world. 

The portal stays open for 1 turn, after which it closes, possibly stranding any characters and creatures in the other world. The World Portal is two-way and while it is in effect a similar portal appears on the destination world that characters and creatures can pass through back to where the spell was cast. If the precious archway has survived, it can be used again for a second casting to enable the travellers to come back. Once the caster has opened a World Portal to a location on another world they get an additional +20% chance of success of opening a new World Portal connecting to that same location. So if the caster has a detailed description and successfully opens a World Portal on the correct world (70% chance) , any subsequent World Portals to that world and that location are at 90% chance of success (70% + 20%). If the caster steps through the portal even briefly and observes  the destination portal's surroundings before stepping back through before the portal closes, the chance of success for that caster reopening the portal at that location goes up to 100% (80% +20%). 

image by Larry Elmore, source

Worlds beyond Mystara

I'm not going to issue any hard and fast rules - this is really up to the DM. You can go homebrew, use  published D&D worlds or worlds from other game systems. You can even include a mythical version of Earth. The limits are really your imagination (and how much work or improvisation you are ready for).  My inclination is to stick to worlds that are compatible with (or at least not too difficult to convert to)  the rules that you are using - in my case B/X D&D. Worlds that spring to my mind as destinations for World Portal spells include:

• Tekumel - world of the Empire of the Petal Throne, created by MAR Barker
• Thunder Rift 
• Alphatia, the previous homeworld of the Empire of Alphatia
• Arduin - created by David Hargrave
• Titan - Home of the Fighting Fantasy books by Ian Livingstone and Steve Jackson
• Averoigne - setting for weird fantasy created by Clark Ashton Smith and also featured in X2: Castle Amber
• Kaelaross - My own homebrew world detailed in the early posts of this blog
• Gamma World - Radically different sci-fi setting but the rules are quite similar

If you as DM are willing to stretch the definition of World and allow the World Portal spell to reach other settings connected to Mystara then it could be a useful spell for reaching Dave Arneson's Blackmoor setting (same planet but different continent and different century), or the Hollow World setting (same planet, but on the inside rather than the outside), or one of the planets or moons of Mystaraspace. 

This spell is 9th level so it cannot be made permanent by the Permanence spell, though if the DM wanted, there might be other ways of keeping the World Portal open for longer than 1 turn (perhaps a carefully worded Wish spell, or Immortal intervention). Characters stranded on a different world are in a difficult situation but not necessarily lost forever. Solutions that spring to my mind include:

• If the caster is also stranded (they went through the world portal but did not return home when the spell expired) then casting a second World Portal spell to get back home requires two things: that the caster has the spell memorised (probably involving studying from the caster's spellbook - was that brought through the portal as well?)  and acquiring a suitable archway for a second casting. Getting the right world is an automatic success. 
• A Wish spell is powerful enough to bring any creatures back to their home world. 
• A Word of Recall spell will work for a high-level cleric, but ONLY the cleric. 
• A Spelljammer ship would carry the PCs back home, but that does depend on whether the destination world has spelljamming visitors. Even if the PCs do find one, it would be an epic voyage across the Crystal Spheres, like Homer's Odyssey. 
• Other magical gates and portals do exist - both Gygax and Greenwood had them in their home worlds (Greyhawk and Forgotten Realms). Whether they take the PCs back to the correct homeworld is another matter...

Image by Larry Elmore, source