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The Demiplane spell lets you create a permanent room 30 feet in any dimension as a separate plane, which would then be smaller than the Tarrasque.

On the other side I may polymorph a Tarrasque into a pot and transport it in my demiplane.

What could happen when, after one hour, the polymorph spell ends?

A typical ruling here could be to eject the creature to the nearest unoccupied space (maybe letting it take some damage, like a mishap when teleporting), but in this case there wouldn't be any "nearest space" where to put it!

Would then be a legitimate ruling to teleport it to the ethereal plane? Or should it just die? Or should I maybe just extend the duration of the polymorph? In other editions they say that when a creature would occupy an illegal position, it should be moved to the last legal position instead.

It is not clear to me where should I put the monster, or what would happen to him.

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The nice answer by Mindwin handles the particular example well; but it partially sidesteps the problem by stating that any creature can squeeze in a 15×15 space, let alone a 30×30×30 volume. This is a correct statement as far as rules are taken as written. It also makes sense if we assume that D&D designers took real-life creatures as a means of gauging the possible sizes of living beings in general. The largest creature that has ever lived on Earth is the blue whale. It can be 170 tonnes in weight, assuming the density of water, that corresponds to over 6000 cubic feet. So even a blue whale would occupy about 20% of the total volume created by the Demiplane spell.

However, this answer might not satisfy those of us who play the game less from a gamist perspective, but more from a simulationist one. A blue whale is 100ft long, so while its total volume is small enough to fit, you would need to fold the poor animal. What happens in that scenario is up to each DM. The folding might harm the animal (possibly killing it) or the pressure by its body might be large enough that it tears the boundaries of the demiplane. In the case of the tarrasque, it being a siege monster and being able to consume just about anything, it would be logical to assume that it would attack the walls. Finally, we can always imagine magical creatures that are larger than whales or tarrasques; for example rocs have wingspans longer than 200ft; assuming the picture in the Monster Manual to be correctly proportioned, its abdomen alone is comparable in size to that of the tarrasque.

Hence, if we assumed there were a creature that would not fit into the space generated by the Demiplane spell (as the title of the question states), and assuming that it can still be targeted by True Polymorph (I write this as we have already gone beyond RAW creatures), what would happen at the end of the polymorph's duration is up to individual DMs. I would personally rule that (1) the creature will get compressed and harmed and possibly killed; (2) if the creature's 'pressure' is large enough to break the wooden/stone walls of the room, the demiplane's boundaries would be ruptured. In that case, just like a ruptured Bag of Holding, the contents could spill into the Astral Plane. (Or to the Ethereal Plane if you assume demiplanes are in the ethereal following the conventions of earlier D&D editions.)

The creatures would not be ejected to the nearest unoccupied space as the demiplane is not on the prime material plane. We read the casting of the Demiplane spell creates the space, and that we can connect to such spaces with future castings of the spell from other locations, this implies that the demiplane is not tethered to any specific location in the prime material.

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  • \$\begingroup\$ or it just folds the creature killing it? \$\endgroup\$ – Voromir Kadien May 25 '18 at 5:57
  • \$\begingroup\$ "In that case, just like a ruptured Bag of Holding, the contents would spill into the Ethereal Plane." - Per the Bag of Holding's spell description, the contents spill into the Astral Plane, not the Ethereal. \$\endgroup\$ – V2Blast May 25 '18 at 5:59
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    \$\begingroup\$ @V2Blast: thanks for the correction. \$\endgroup\$ – ZwiQ May 25 '18 at 6:24
  • \$\begingroup\$ @VoromirKadien: I had mentioned that it can harm the being, but I added killing in parentheses. \$\endgroup\$ – ZwiQ May 25 '18 at 6:26
  • \$\begingroup\$ I think logically it should kill the creature assuming there is enough space to store the creautre in whatever form it ends up in. Taring wholes in the fabric of "reality"should only happen if there is more matter than space. \$\endgroup\$ – Voromir Kadien May 25 '18 at 6:31
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The tarrasque is trapped in the demiplane, if the door is already gone. It is not harmed, though.

Even if the door is still there, it probably cannot go through. The door is probably too small for a gargantuan creature to get through.

The door is large enough to allow Medium creatures to pass through unhindered.

Your assumption that the tarrasque is bigger than the demiplane is false. The demiplane is a 30x30x30ft cube. The Tarrasque is a gargantuan monstrosity, and its space is 20ft by 20ft.

Even if the space was 1/8th of that (a 15x15x15 cube), the tarrasque could stay inside neatly squeezed.

Even if a creature would be bigger than a 20x20 space, it is still only gargantuan. Any creature can fit/squeeze in a 15x15 space (being the space "one size smaller" than gargantuan).

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    \$\begingroup\$ Actually gargantuan refers to creatures 20x20 and bigger, in fact the MM(p.286) says that the tarrasque is 40ft. tall and 70ft. long! \$\endgroup\$ – Dinisaur May 24 '18 at 23:40
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    \$\begingroup\$ @Dinisaur that doesn't really change the answer though since, according to the rules, it is still gargantuan and thus can squeeze into a 15x15 square regardless of exact dimensions. \$\endgroup\$ – Rubiksmoose May 25 '18 at 1:05
  • \$\begingroup\$ @dinisaur see the last paragraph. \$\endgroup\$ – Mindwin May 25 '18 at 12:38

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