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The following situation happened towards the end of our last session(I am the DM): A player cast Mordenkainen's Magnificent Mansion, so they could take a break to heal up their comrades and rest a bit.

Then, that player cast the Mansion again (after a long rest) and placed the portal inside the first mansion. Everyone went through before the first mansion disappeared, and expected to return to the main overworld again after another 24 hours.

But technically, the Mansion only expels creatures, not portals to extradimensional planes, so shouldn't they be thrown out where the first mansion was, an extradimensional space?

I am not gonna do this to them now since this would be the end of the story and not something anyone expected, but still: Under RAW, would my players be back to the extradimensional space where the first mansion was?

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    \$\begingroup\$ Welcome to RPG.SE though it looks like you have been around for a while! Take the tour if you haven't already, and check out the help center for more guidance. Good Luck and Happy Gaming! \$\endgroup\$ – Someone_Evil Apr 30 at 10:29
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    \$\begingroup\$ related: stacking bags of holding \$\endgroup\$ – Phil Apr 30 at 13:05
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    \$\begingroup\$ Upvote for magnificent title. \$\endgroup\$ – Stackstuck Apr 30 at 22:28
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    \$\begingroup\$ @Stackstuck: Originally it was just "matryoshka mansions", but it wasn't clear that it was referring to the spell and I couldn't resist the alliteration ;) \$\endgroup\$ – V2Blast May 1 at 0:05
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The Mansion no longer exists

The first line of the spell reads:

You conjure an extradimensional dwelling in range that lasts for the duration.

(emphasis added)

This sentence implies that the mansion exists only for the duration, and ceases to exist afterwards. The ethereal nature of the mansion's contents (conjured servants cannot leave the mansion, and conjured objects vanish if removed) attest to the ephemeral nature of Mordenkainen's Magnificent Mansion.

However, you have noted a few holes in the logic of the spell. The spell has no provision for what happens if you open portals inside the mansion, or even for unattended objects in the mansion at the end of the spell. By the rules as written, the players in your scenario would not return to the first mansion when they attempt to exit the second mansion, but where they do end up is unclear and has no RAW guidance.

I see several possible options for when the party attempts to exit the second mansion when the first mansion expires. Pick the one (or make up your own) which you think would make the best adventure for your party, although remember that whichever option you choose sets a precedent for how this spell will work in future.

  • The party exits back on the Material Plane where the first mansion had its portal. This is a minimal fuss solution if you don't feel like making an adventure from their shenanigans. It has minimal exploit potential, but is also the most boring option with no consequences for carelessly bending space as they have.

  • The party finds that they cannot exit the second mansion; they're trapped! This technically only delays the outcome, as the second mansion would expire eventually. The players would need to figure out how to exit the second mansion before it expires or else face an uncertain fate (such as being dumped in the Astral Plane; see below). I would allow a special casting of Mordenkainen's Magnificent Mansion, accompanied with an Arcana check, to re-establish the portal and let them escape, although there are no rules to support this escape plan. Plane shift should work as an escape, though. If your players have access to appropriate spells, then this makes for a single encounter, but if they don't then they may be forced to sit and await their fate. This option could be exploited by your players to create some kind of prison, although with 7th level spells there are probably better spells for this sort of thing.

  • The party exits into the Astral Plane; adventure ensues! A look at the description for a bag of holding (DMG p.153) indicates that dumping things into the Astral Plane is the option of choice when it comes to breaking extradimensional spaces. And putting a portal to Mordenkainen's Magnificent Mansion within another Mordenkainen's Magnificent Mansion which then disappeared is exactly the sort of breaking of extradimensional spaces which deserves to open a rift to the Astral Plane. You can take a look at pages 46-48 of the Dungeon Master's Guide for a description of the Astral Plane, including the existence of 'colour pools' which serve as gateways out of the Astral Plane and implies your characters will not be stuck forever (although, as DM, you are free to modify the Astral Plane to suit your campaign setting). This option presents you with the opportunity for a full adventure, which you can make as long or brief as you like, and your players will likely remember the time their messing with space caused an accidental detour through the Astral Plane. This could potentially be exploited as a means for dumping people into the Astral Plane, but plane shift is also a 7th level spell so this is not unbalanced.

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    \$\begingroup\$ The party was just going to let the second mansion expire, so I guess I'm gonna go with "The portal to the second mansion was also ejected from the first mansion, so it is now back on the material plane, close to where you started." Thanks for the effort of writing this down though, I may take some inspiration for later adventures. \$\endgroup\$ – Alexander Wolters Apr 30 at 11:41
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    \$\begingroup\$ @AlexanderWolters My personal preference would be to throw the party through the Astral Plane, but it's your party and your call. \$\endgroup\$ – BBeast Apr 30 at 11:43
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    \$\begingroup\$ @Miniman With the benefit of hindsight, that would probably be true, but lacking such hindsight having them exit into the Astral Plane instead of their intended destination when they attempt to leave the second mansion is also passable. \$\endgroup\$ – BBeast Apr 30 at 13:28
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    \$\begingroup\$ @Miniman: The MM spell doesn't mention any no-nesting restrictions the way a Bag of Holding / Portable Hole does, and you can bring such portals to extradimensional spaces into a Mansion without any problems. I don't see any reason why opening a new portal inside an existing mansion would trigger a rift. I think BBeast's judgement of expelling the party into the Astral Plane when the second mansion expires makes much more sense. (Or possibly when the outer mansion expires, if "destroying" the outside of the entrance/exit portal destabilizes the spell.) \$\endgroup\$ – Peter Cordes Apr 30 at 23:01
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    \$\begingroup\$ @PeterCordes I have no interest in having an extended argument over this extremely trivial point. \$\endgroup\$ – Miniman May 1 at 0:51
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You choose where its one entrance is located.

It has one entrance.

There are no words that say what happens if that entrance is destroyed. The spell could continue to function, or it could end.

If the spell ends when the entrance is destroyed, the result of the matryoshka inner mansion is that when the outer mansion is destroyed, so is the inner entrance, which expels its contents, which then in turn gets expelled. Ie, the inner mansion doesn't extend your stay.

That is by far the simplest solution.

If the spell continues when its entrance is destroyed things get interesting.

A 1 layer mansion with its entrance destroyed, arguably "nearest the entrance" still makes sense as the spot nearest the location where the entrance was. So the inhabitants are trapped until the end of the spell, at which point they are expelled.

In a 2 layer mansion, we need to work out where the portal is, then find where the "nearest" open space is. This requires you, as a DM, defining how the extradimensional space works.

Is it in the astral plane? Is it a brand new plane? If so, where is the new plane, is it a bubble off the prime plane, or somewhere else? Is there a plane of mansions that this spell accesses? Is there a devil who harvests spell energy in exchange for setting up these mansions?

Wherever it is, you then define where the "nearest open space is".

Personally I'd go for the plane of mansions

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