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Visiting the lost mansion of a conjurer, where portals were opening for a few seconds, a sorcerer tried to keep open one that led to the Elemental Plane of Fire blocking it with an Immovable Rod.

I'm interested in what would happen in order to preserve an internal coherence regarding physical/magical laws that govern the world.

Here are some thoughts that may explain what I mean:

  • The object may just remain on one side, and occupy the space that it would have occupied without the portal (e.g. if the portal was lying on a wall, the the rod would have been stuck in the wall). However, this raises another question: on which side of the portal would the object end up?
  • The object may just split together with the spacetime as the portal closes, so each part would remain on the side of the portal where it was, effectively cutting the object. In my opinion, however, this mechanic would provide an easy way to cut objects that where supposed not to easily break.
  • I may imagine a pressure-like force that acts on objects near/in the portal, inversely proportional to the surface of the portal, so that objects are eventually pushed/sucked on one side. This is an incremental version of the first idea, and it's my favourite at the moment.

So the question is the following: what would happen if I put an Immovable Rod (or in general an object) into a closing portal?

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    \$\begingroup\$ What sort of portals? Were they created with an existing spell or magic item? If not, it's just pure DM call how their portals work. \$\endgroup\$ – Tin Man May 22 '18 at 19:11
  • \$\begingroup\$ @Walt We may suppose these portals have been created with the same mechanics as gate and arcane gate, depending on where they were bringing. \$\endgroup\$ – Dinisaur May 24 '18 at 8:38
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From readings of spells like Arcane Gate and Dimension Door it seems like the idea of an instantaneous moment between entering and exiting has been simplified (or glossed over) for the sake of simplifying game mechanics. The spells tell you what happens when you begin the interacting action and the conclusion of it, not what happens between the two. Because of this I would agree with NathanS in that it is up to the DM.

To offer another idea for resolution:
Reading through other sources like the DMG and SCAG we see that in the 5e cosmology a lot of different planes overlap and occupy similar "spaces." If this scenario played out in one of my campaigns I would rule that after the portal closed the Immovable Rod would now exist in both planes simultaneously, looking like it was cut in half (or however far it was pushed into the portal) but was actually still a continuous piece. This means that part of the rod is floating in one plane and the other part is floating in another. Interacting with the item after the portal closes could play out in two ways:

  1. The only way to move or interact with the rod would be to recreate the scenario that got it stuck in the first place and then deactivate it or move it
  2. It can be deactivated and moved as normal, but only the portion that is on the same plane as the interacting entity can be seen/manipulated/affected/etc. Here the part of the rod in the other plane would move in relation to what is happening on the other side. I wouldn't allow it to be used as an "invisible weapon" or something similar unless the creature it is being used against also exists in both planes simultaneously
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  • \$\begingroup\$ I like this new point of view! It would be quite hard to recreate the portal in the exact same spot though... Thumb up for the creativity anyway :) \$\endgroup\$ – Dinisaur May 23 '18 at 21:04
  • \$\begingroup\$ Half stuck in the plane of fire... so a white hot glowing metal half of a rod hanging in the air... good luck deactivating it. \$\endgroup\$ – user47897 Mar 12 at 20:04
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Taking a look at the description from 5e spells that open portals to other planes such as the 9th level conjuration spell Gate:

The portal has a front and a back on each plane where it appears. Travel through the portal is possible only by moving through its front. Anything that does so is instantly transported to the other plane, appearing in the unoccupied space nearest to the portal.

It is apparent that objects can't stay inside the portal aperture. An Immovable Rod would end up the same way. You would have to position the Immovable Rod initially by sticking it into the portal before pressing the button, but the portal would not give you the chance to do so and would immediately transport the Immovable Rod (and quite possibly you) to the plane on the other side.

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    \$\begingroup\$ Good answer. You won't see the "Stargate" scene with O'Neill's hand sticking in the portal in DnD... You need to see the portal as a magical effect that transports everything it touches to its target plane. From the outside, you see a person approaching the portal, and then ... zip, gone. \$\endgroup\$ – Anonymous May 23 '18 at 5:32
  • \$\begingroup\$ What if the thing traveling through the portal doesn’t fit in the location the “back” resides? For example: the front of the portal is in a banquet hall, the back is in a broom closet, and a boat just touched the entrance. \$\endgroup\$ – zero298 May 23 '18 at 12:27
  • \$\begingroup\$ @zero298 Interesting question. This answer only deals with interplanar portals the OP queried. However, I did find an answer to an older, similar question here. Ask a question if that doesn't help you. \$\endgroup\$ – Tenryu May 23 '18 at 12:39
  • \$\begingroup\$ @zero298 I'd imagine it gets a little...messy... \$\endgroup\$ – Ynneadwraith May 23 '18 at 12:41
  • \$\begingroup\$ @Tenryu On the other side, as you pointed in your link, the Arcane Gate spell sounds more like the stargate portal... It may not be clear from the question but I think it's still relevant, since in the said tower portals were opening to many different locations, even different rooms of the tower \$\endgroup\$ – Dinisaur May 24 '18 at 8:23
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Up to the DM

So if you're the DM, it's up to you. Since such an interaction is not specified in the description of the Immovable Rod in the DMG, there is no official guidance on this, so the DM will need to rule the most plausible outcome according to how they believe the situation should play out.

Consider the Bag of Holding (DMG page 154, where certain interactions are anticipated:

Placing a bag of holding inside an extradimensional space ... instantly destroys both items and opens a gate to the Astral Plane.

If there was a specific intended outcome, you would expect to find similar text under the Immovable Rod entry, but there isn't, so it's really up to the DM.

The fact that you've already listed some ideas implies you're the DM, and the fact that you've specified that one of your ideas is your favourite, that's probably the one you should use if you are the DM, but another DM may decide differently and that's fine too.

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