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!)?
Best Answer
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:
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.