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Some context: myself, and another dungeon master were discussing Daern's instant fortress and its interactions with a bag of holding.

  • My stance was that the instant fortress is essentially utilizing a very powerful enlarge/reduce spell.
  • And his stance was that the instant fortress is an extra-dimensional space, and thusly, would not interact positively with a bag of holding, or any other massive capacity storage device.

We were discussing this due to a player wanting to create a mystical item that combines the effects of the instant fortress and the bag of holding to allow for the fortress to have things inside of it even when reduced.

Is Daern’s Instant Fortress considered an extra-dimensional space?

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    \$\begingroup\$ I would suggest that only a portion of the Fortress be extradimensional so that you could store all of your furniture in the Closet of Holding, but you could still bring your Bag of Holding and Portable Hole into the fortress. Just don't throw them in the Closet. \$\endgroup\$ Sep 13, 2023 at 15:11
  • \$\begingroup\$ Huh, reading the question and the answers, people seem to interpret Instant Fortress as being able to return to the cube form. But reading the text, it just says you "dismisses it", which usually means just making it vanish, right? And since the cube grows into the fortress, dismissing the fortress means dismissing the cube too. I.e., so far I've been interpreting Instant Fortress as a single use item. \$\endgroup\$
    – justhalf
    Sep 13, 2023 at 18:33
  • \$\begingroup\$ @justhalf I think you overinterpret what "dismiss" means here. It likely just means dismiss the fortress form, so it returns to cube form. The fortress is not designated as a consumable item (p. 141 DMG), that is used up when activated, like a potion or scroll. While there may be some bona fide wondrous items that have only limited uses before they are used up, Instant Fortress does not make any mention of being limited in uses. Other non-consumable wondrous items that have a limited number of uses, such as a robe of useful items, say so much more explicitly. \$\endgroup\$ Sep 14, 2023 at 13:51
  • \$\begingroup\$ that makes most sense RAI of course, and that was my first intuition before reading the text in details, but just got stuck in that "dismiss" part. Figurine of Wondrous Power, for example, explicitly state that it "reverts to its figurine form", while that wording is not used here, so that's why I was RAW-confused even though I am RAI-clear. The revert makes sense for the rarity and the cost too, more than a single-use. \$\endgroup\$
    – justhalf
    Sep 14, 2023 at 15:37

2 Answers 2

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No, it is not an extradimensional space

Instant fortress does not say it is or contains an extradimensional space. It says:

You can use an action to place this 1-inch metal cube on the ground and speak its command word. The cube rapidly grows into a fortress that remains until you use an action to speak the command word that dismisses it, which works only if the fortress is empty.

The cube rapidly grows into a fortress. That is, the cube already is in this dimension, it just grows. Compare this to other objects like bag of holding, handy haversack, or portable hole: they say that they are larger on the inside than from the outside, and they also reference extradimensional spaces, here for example from handy haversack:

This backpack has a central pouch and two side pouches, each of which is an extradimensional space. [...] Placing the haversack inside an extradimensional space created by a bag of holding, portable hole, or similar item instantly destroys both items and opens a gate to the Astral Plane.

If you homebrew a new item, that combines the features of one of those with instant fortress, for example by allowing items to remain within the fortress when you shrink it by putting those items into an extradimensional space, how that item works is up to you and the DM you work with. One way could be to use similar language (e.g. by putting the "instant fortress of holding" into another extradimensional space, or a bag of holding into it when it is shrunk, they both get destroyed and open a gate to the astral plane.)

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The fortress cannot be put away if there are things inside it.

The item description of Daern’s Instant Fortress states:

The cube rapidly grows into a fortress that remains until you use an action to speak the command word that dismisses it, which works only if the fortress is empty.

You must clean out the fortress completely before you can put it away, so it cannot be used as storage.

As for the fortress being an extradimensional space, the description says nothing of the sort, so it is nothing of the sort.

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    \$\begingroup\$ I think they know that they normally cannot leave items in the fortress when they shrink it. They want to "create a mystical item that combines the effects of the instant Fortress and the bag of holding to allow for the fortress to have things inside of it even when reduced". \$\endgroup\$ Sep 13, 2023 at 4:17
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    \$\begingroup\$ Quite honestly the power of compressed storage is much lower today than it was in the GP = XP period of D&D's history, and it seems like a lot of the rules around carrying capacity are still sort of clinging to the idea that storage space is innately valuable. That said, I think the important implication would be less about storage and more about whether the fortress can be compressed with people and animals inside. The "Bottle Fortress of Kandor" seems like a potentially problematic exploit. \$\endgroup\$ Sep 13, 2023 at 17:14
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    \$\begingroup\$ @TigerGuy The genie’s vessel is an extradimensional space \$\endgroup\$ Sep 14, 2023 at 15:53

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