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The Instant Fortress magic item has the potential to deal a great deal of damage (~55 on a failed save, more than a 9th level Fireball spell). Many adventurers who acquire this item attempt to weaponize it. The text for the Instant Fortress reads:

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.

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Each creature in the area where the fortress appears must make a DC 15 Dexterity saving throw, taking 10d10 bludgeoning damage on a failed save, or half as much damage on a successful one. In either case, the creature is pushed to an unoccupied space outside but next to the fortress. Objects in the area that aren't being worn or carried take this damage and are pushed automatically.

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In order to activate it, "You can use an action to place this 1-inch metal cube on the ground and speak its command word". Rules as written, it seems like the activator will be caught in the "blast radius" of the expanding fortress. Is that true? If so, are there any ways for the activator to avoid the "blast"?

A bit of background: I usually play at Adventurers' League tables. Most of those tables try to play as close RAW as possible. My motivation for asking this question is to gain a better understanding of how an Instant Fortress functions according to RAW. If RAW is ambiguous, I would greatly appreciate a logical explanation of how it should reasonably function.

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    \$\begingroup\$ Are you assuming that the cube is at the center of where the fortress will ultimately be located? \$\endgroup\$ – V2Blast Apr 20 at 3:24
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    \$\begingroup\$ @V2Blast Yes, I assumed that the cube will be at the center of where the fortress will be located. Also, as a small note, I usually play Adventurers League, so I'm trying to get a RAW understanding of how Instant Fortress functions, particularly in combat. However, it seems like much of its behavior is decided by DM ruling. \$\endgroup\$ – Joshua Apr 20 at 3:42
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    \$\begingroup\$ If you're interested in the rules as they apply to AL specifically, you should probably mention that in the question and edit in the [dnd-adventurers-league] tag - sometimes AL rules differ from the regular 5e rules, and the rest of the time AL is prevented from deviating from the regular 5e rules. \$\endgroup\$ – V2Blast Apr 20 at 6:43
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The fortress is probably not designed to kill its user

You are correct that the text is ambiguous, in that it doesn't specify how or in what direction it expands from a 1-inch cube into a fortress 20 feet on a side. However, since it is a magic item that was designed by an intelligent mage who presumably intended to actually use their creation, it is reasonable to assume that it functions in such a way that its normal use is not lethal to the user.

To that end, the DM will have to make a ruling about how the fortress unfolds. Personally, my ruling would be that the fortress unfolds away from the user in such a way that the entrance door ends up right where the cube was placed, facing toward the user with the fortress entirely behind it. Another reasonable ruling would be to use the same rules as for placing a cube-shaped area of effect, with the user choosing the space for the fortress to expand into in the same way they would choose the area of effect for a thunderwave spell.

Item descriptions usually explicitly call out effects harmful to the user

As further evidence that the Instant Fortress isn't supposed to harm the user, I would point out that when an item has a negative effect for the user, that effect is usually called out explicitly, for example by describing it as a curse. (I won't cite any specific examples, since they would all be spoilers, but you can search the DMG for examples.)

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    \$\begingroup\$ i'm going to put -1 because to activate the cube, you need only say the activation word, The item has no specified range, meaning that one who is sufficiently intelligent can place the cube down, take 4 paces back and say the keyword to avoid clobbery.. A user who doesn't take the simple precaution to get out of the way, will get the results, including the damage. \$\endgroup\$ – tuskiomi Apr 20 at 4:16
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    \$\begingroup\$ @tuskiomi You also need to put the cube on the ground as part of the action so it's not only saying the activation word. \$\endgroup\$ – Ling Apr 20 at 7:31
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    \$\begingroup\$ @Ling which is strange as both speaking and dropping items are normally free actions. \$\endgroup\$ – tuskiomi Apr 20 at 7:32
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    \$\begingroup\$ @tuskiomi - By that (command-word only) logic, it also doesn't say that the cube would have to be anywhere nearby to begin with. You'd be able to leave it somewhere inconspicuous, and then trigger it days or weeks later. \$\endgroup\$ – Bobson Apr 20 at 22:00
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    \$\begingroup\$ @tuskiomi - I was thinking you'd spend that one action weeks later. But it's a moot point since setting it down is actually part of the activation. \$\endgroup\$ – Bobson Apr 20 at 22:15
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The design intent is for the fortress to expand next to the user, with the user safely out of the way. Earlier editions were a bit more explicit about that; for example, in 3.5, the rules said

The fortress springs up in just 1 round, with the door facing the device’s owner. The door opens and closes instantly at his command. People and creatures nearby (except the owner) must be careful not to be caught by the fortress’s sudden growth.

See the official rules (RTF, archived copy), or the less official but much more convenient d20srd.org.


The 5e rules still imply that the owner ends up outside the fortress:

When activated, the tower has a small door on the side facing you.

although this could theoretically be interpreted as "you get smashed by the fortress, and end up next to the door after smashing".

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    \$\begingroup\$ Oh, I didn't even notice the bit in the text about the door facing you in the description. I just came up with that based on it being the most obvious way to place it unambiguously. \$\endgroup\$ – Ryan C. Thompson Apr 20 at 3:22
  • \$\begingroup\$ It is only an implication, but also means that the fortress can't magically grow around the user with the user now inside. For the door to face someone, that person must be outside. \$\endgroup\$ – Mooing Duck Apr 20 at 22:11

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