Situation: a mage casts Demiplane to create a wood-walled plane, then torches it before pushing a target into it with the ending of the spell's duration. This essentially seals the poor target into an eternal fire dimension.

The 8th level spell Demiplane creates a small plane, 30×30×30 feet, made of wood or stone. When the spell's duration ends, the entrance disappears and anything within is trapped in this alternate dimension.

Obviously, in reality, such a situation couldn't last forever (the fire would run out of air and extinguish quickly). Is this possible in D&D, however, given the limits of the spell and the game's limited rules coverage of how fires work? Moreover, what would happen to the Demiplane once its walls were consumed (assuming they even could be, as I don't recall rules for wood being consumed!)?

You can't set the plane on fire

Since D&D 5e is about "rulings, not rules" it would ultimately be up to the DM. But for me, the sticking point is:

When opened, the door leads to a demiplane that appears to be an empty room 30 feet in each dimension, made of wood or stone.

The demiplane is not made of wood or stone, it just looks that way.

This makes sense when you think about cosmology. The walls are the extent of the demiplane, and you could not burn them down any more than you could, for instance, enlarge a stone demiplane with a pickaxe. Since the walls cannot be consumed, no fire can catch in the first place, since fire requires fuel.

But you can bring in flammable objects

Nothing is stopping you from filling the plane with wood or oil beforehand, and igniting that before chucking in the hapless victim. That will last for as long as there's fuel (oxygen and wood...or flesh).

What happens in this scenario is also up to the DM, but as any firefighter will tell you, the biggest threat from fire is the smoke, not heat. Even before the oxygen runs out, you're breathing in super-hot carbon dioxide and smoke, so you pass out from the pain of your burning lungs. I hope your victim had a chance to take a deep breath before falling in, for the "holding your breath" rules to apply. Then they have (1 + Constitution modifier) minutes before suffocating anyway.

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    Appearing to be wood does not preclude something from actually being wood. Many wooden objects IRL actually appear to be made of wood. – indigochild Aug 22 '16 at 21:19
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    @indigochild It's true. That would be persuasive if the answer didn't provide further reasoning for why a plane's boundaries can't be consumed/mined/extended. – SevenSidedDie Aug 22 '16 at 21:29
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    @SevenSidedDie The first bolded section doesn't make sense without that platform. The walls, even if they coincide with the boundaries of the demiplane, may still be flammable. – indigochild Aug 23 '16 at 14:06
  • I would say those walls don't actually exist at all. It's not wood/stone walls up against the edges of the pocket dimension, but something like an illusion of wood/stone at the very boundary of the plane's existence. So burning them wouldn't work, and walking over the wood 'floor' wouldn't cause it to creak. This isn't a room trapped in a different plane, but the entirety of the plane itself - which is empty. – James Alday Aug 25 '16 at 18:49

It is the Referee's Call

Because of the lack of defined terms all we know for sure is that the spell opens a door that leads to a 30' by 30' by 30' room where the wall are made of stone or wood. There is a door under the caster control that can be removed. That it can be linked too again with a new casting of the spell.

The rest relies on what you know about various fantasy tropes. For example the fact it mentions that it is a plane lead to a number of different tropes like a pocket dimension. As you can see from the linked article there are number of ways of looking at what a pocket dimension is and how it can be interacted with. The main thing that unites the concept is that it is a small space and it is "elsewhere" outside of normal life.

As for your question whether the walls can be set on fire, a case can be made for either interpretation. I have to disagree with @SPavel in that it not clear what the word appears refers to. One reading is that the room appears to be empty. But SPavel's reading is reasonable too. Which lead back to my answer that it is the referee's call.

But I will say that it is my opinion that most people's answer will center on the following.

  • The Demiplane is 30 by 30 by 30 and can't be expanded by digging
  • That if the wall are of real wood setting them on fire will kill anybody in there rather quickly for the reason SPavel gives about how fire works in real life.
  • But it just as likely that the ruling will be that the wall are magical wood and stone and can't be damaged.
  • It will split 50-50 on whether you run out of air or not. Some people will rely on the how the portable hole work. Others will rely on the pocket dimension prison trope and have the air magically replenished.
  • Most will probably say you starve or die of thirst given time unless there is food and water inside.
  • A few may consider it a form of stasis with everybody inside frozen in time. Which means the fire is frozen when the door is removed.

    I suggest you and your referee look over the pocket dimension trope and come up with something that make sense for future campaigns.

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