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AFAIK, demiplanes are 30-ft3 cubes placed somewhere on the Ethereal Plane.

I’ve been thinking of a few ideas about demiplanes, but I’m not sure how demiplanes relate to the Ethereal Plane they’re in, so I’m not sure how or whether they work.

My ideas involve:

  • Creating a window to the Ethereal Plane on the demiplane; if this is possible, what would happen if you open it?
  • Creating two adjacent demiplanes and connecting them to make a larger space.
  • Creating two demiplanes near each other and connecting them with a bridge.

Are these things possible using game mechanics (without including use of the Wish spell)?

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    \$\begingroup\$ So, that's not what a demiplane is. Do you want us to assume that's what a demiplane is and answer the question that way or do you want us to explain what demiplanes actually are and their actual interactions with the Ethereal plane? \$\endgroup\$ – Please stop being evil Jun 8 '19 at 17:52
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    \$\begingroup\$ Also, Hi! Welcome to RPG.se :) Your initial question (besides the ambiguity caused by your error re: demiplanes) looks like a good fit for the site, but feel free to check out our help center and take the two-minute tour when you have time; our Q&A site is somewhat different from typical forums. \$\endgroup\$ – Please stop being evil Jun 8 '19 at 17:55
  • \$\begingroup\$ Well, that's a shortened description. I'd like to know how would it function with actual demiplanes. I think this type of miscellaneous is up to DM, but someone might know some specific rules. \$\endgroup\$ – user10286305 Jun 8 '19 at 17:56
  • \$\begingroup\$ I think this question should be tagged as "Homebrew" given the fact that it uses a system of planes that differs from the traditional canon. \$\endgroup\$ – Rykara Jun 14 '19 at 16:58
  • \$\begingroup\$ @Rykara: It's not homebrew; it presents OP's understanding of how planes and demiplanes officially work, which may or may not be mistaken, and asks whether certain interactions with them are possible within the rules. If it is mistaken, that should be pointed out in an answer. \$\endgroup\$ – V2Blast Jun 14 '19 at 22:13
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What is a demiplane?

Demiplanes are defined as "small extradimensional spaces with their own unique rules" (in the PHB's appendix C, p302, they're also described with slightly different wording in the DMG, p68). So any small plane can be called a demiplane, and you shouldn't expect them to all be the same or have even have many rules in common. The whole Ravenloft campaign setting is contained within the Demiplane of Dread, so even "small" is relative.

Creating demiplanes with spells

In your question, you seem to be specifically referring to demiplanes created by the 8th spell Demiplane (PHB p231, in basic rules, and online here). It's worth noting that that's not the only way to create such a plane. Several other spells also create demiplanes, some of which are much simpler. For example, the 2nd level spell Rope Trick (PHB p272, in the basic rules, and online here) creates a temporary demiplane at the top of the rope that is only defined well enough to say that it can fit up to eight medium-sized creatures. The 7th level spell Mordenkainen's Magnificent Mansion (PHB p261, also in the basic rules as Magnificient Mansion without the proper name, and online here) lets its caster conjure up a whole building with furnishings and invisible servants that exists in a temporary demiplane.

The demiplanes created by the Demiplane spell have some specified characteristics (30ft cube, bounded by wood or stone walls, and since you can come back to the same demiplane, it's presumably permanent), but other characteristics are unspecified (like how a created demiplane relates to the Ethereal Plane). There are no official rules yet in D&D 5e for joining demiplanes, or many of the other things you have asked about. However, many of those ideas ripe targets for homebrew between a PC and their DM.

Homebrewing special rules for demiplanes

If you're the player of the PC who wants to play around with demiplanes, I suggest taking to your DM about exactly what you want to achieve. Together you can work to come up with an appropriate set of rules. For ideas, consider looking at earlier editions of D&D and Pathfinder, where you'll find playtested and at least somewhat balanced spells and other rules (though as with any homebrew, your DM will need to think about how well outside concepts will fit into your specific game). This might be one good starting place!

Your DM might tell you that you can just do whatever you have come up with, as part of casting Demiplane. Or maybe you will eventually be able to learn to do it, but only after a long-term project, as you (or NPC wizards that you hire) will have to do a bunch of research and experimentation first. The learning process might be something you come back to several times between your other adventures.

But your ambition to tinker around the defined boundaries of your spells also gives the DM a great hook to send you off on an adventure specifically about this topic! Maybe your preliminary research will tell you that you can't merge two demiplanes without the knowledge of a specific wizard who once lived in your world. But alas, she's been lost in extraplanar space for decades or centuries. Can you find her? If she's trapped on a demiplane with weird rules of physics and time, can you get her free without becoming trapped yourself? Will she agree to teach you if you do rescue her? It could be fun to find out!

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  • \$\begingroup\$ Also, as we are discussing of homebrewing, you should probably point out that much of this is probably addressed (in a play-tested and reasonably well balanced form) in rules from previous editions that can be used for inspiration. Pathfinder, being based on 3.5, is a good place to start since the information is freely available online and well indexed: d20pfsrd.com/magic/all-spells/c/create-demiplane \$\endgroup\$ – cpcodes Jun 14 '19 at 20:33
  • \$\begingroup\$ @cpcodes: Thanks for the suggestion. I've edited my answer to include your suggestions about how a player and DM could go about making homebrew rules based on the official rules from earlier editions. I also made my answer a bit clearer about the fact that 5e doesn't have any official rules for this, at least, not yet. \$\endgroup\$ – Blckknght Jun 14 '19 at 22:42

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