Demiplane inception

[Tiny Coffee Golem]

If you make demiplane B within demiplane A What happens to B if A is destroyed?

[Arbalester]

If a plane is destroyed? And plane B is entirely in plane A? I'd say B is also destroyed... now if B is coterminous or coexistent with A, I'd say no, as B can still exist as a separate plane.

[VRMH]

How can a Plane be "within" another Plane?

[Rune]

You can enter some kind of demiplane (many adventures have that) and once inside, you could create another one (like the one created by Mage's Magnificent Mansion).

If I was DMing, I would choose whatever course of action that provided the coolest story hook.

Example: Adventurer's enter the Mad Wizard's own hellish demiplane. As soon as they kill the villain, the demiplane starts to fall apart. The heroes hastily cast a Mage's Magnificent Mansion or even a simple Rope Trick. Suddenly, they are inside a extradimensional space inside a demiplane.

I'd rule that the extradimensional space persists, but what could be its only way out now exits unto a non-existant space. When the duration ends, the heroes are shut out to a random spot on the Astral Plane, the remains of the Mad Wizard's demiplane floating around. In the time it took for the PCs to get there, some of the ruins have already been occupied by strange creatures of thought, and maybe one of them could have a way out (if it doesn't eat your mind and soul first).

[Ravingdork]

You can't have a plane inside another plane. They exist outside of space by their very nature and as such are always separate and distinct from one another. What you CAN do, however, is have a gateway to one plane exist in another plane.

In such a case, the "interior plane" (as you describe it) would be cut off from all other planes the moment the "exterior plane" was destroyed. It would continue on existing, however, and could be accessed by being with plane shift and similar powers who are aware of its existence.

[Tiny Coffee Golem]

From the lesser demiplane spell;

You create a small, finite demiplane. You must be on the Astral or Ethereal Plane or on a plane that has access to one of those planes (such as the Material Plane) to cast this spell.

So guess it would depend on the cosmology of the universe you were playing in. If you could create a coterminous demiplane to the astral (for example) then you could make one within another, but not twice.

It's fantasy cosmology, so for the sake of arguement, lets just say you can make your demiplane coteerminus to the astral.

[OldManAlexi]

Tiny Coffee Golem wrote:

From the lesser demiplane spell;

You create a small, finite demiplane. You must be on the Astral or Ethereal Plane or on a plane that has access to one of those planes (such as the Material Plane) to cast this spell.

So guess it would depend on the cosmology of the universe you were playing in. If you could create a coterminous demiplane to the astral (for example) then you could make one within another, but not twice.

It's fantasy cosmology, so for the sake of arguement, lets just say you can make your demiplane coteerminus to the astral.

Example time. A wizard uses the create demiplane spell to create a demiplane in the Astral Plane. He travels to his demiplane and casts the spell again to create another demiplane. Usually, this would put the demiplanes next to each other to create one larger demiplane. However, if he knows what he's doing, he could make a second demiplane on the Astral Plane that is coterminous with the original demiplane. Then, using greater create demiplane, he would make a portal from the Material Plane to the first demiplane and another portal between the two demiplanes. If the first demiplane was destroyed, the second demiplane would be unharmed but the portal would be destroyed because it doesn't lead anywhere anymore. Good luck getting off.