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.…
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.…
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.
Best Answer
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.)