Is this a magic item count reducer?

Is it restored to full hit points when it's dismissed and then re-summoned? 

Quote from paperbullet >>

Is it restored to full hit points when it's dismissed and then re-summoned? 

 No

Does it have any AC to consider?

whats the weight while its in the 1 inch cube vs expanded?

This is a very useful item to weaponize. simply cast the cube into the midst of an enemy fray, and utter your magic word! You will summon a 20-square foot wrecking ball, dealing 10d10 bludgeoning damage to any opponent under the fortress!

I question it's use in a space not big enough in a magic protected dungeon...like undermountain...

That’s a great question 

I probably shouldn't give this to a group of level 1 adventurers, right?

Also, is there no limit to the number of times it can be used?

Quote from Helmut_McQuack >>

Also, is there no limit to the number of times it can be used?

 Yes but it can’t regain hit points, so it can get worn away over time and you can’t stop it.

is it just me or is it super hard to find a grid map for this online? it seems like there should be a bunch but I cant find a single one.

As a DM, I generally rule that casting Mending on this causes it to regain 1d10 hit points.

If swallowed by a creature, could I activate this to do massive damage to the creature? I know i would take the damage too, just wondering if this was a viable option.

I know the spell says "place this 1-inch metal cube on the ground" implying that it must touch the ground, so no, my idea wouldn't work. However, it specifically says "place", which in my opinion means holding in your hand as you set it on the ground. People often talk of throwing it as a 20'x20'x30' grenade, which imo, goes against RAW.

I mean, if it leaves your hand and ends up on the ground at rest, it's not much different from placing it. I'd rule that its command word won't take effect until its come to a stop, giving enemies a chance to boot it out of the way if they are concerned about it, but otherwise let the PCs chuck it around.

Say it takes a full six seconds to become inert and no longer moving if thrown. That way it's an action in combat to yeet it, and next round expand it. That gives your monsters a round to make an arcana check and boot it away.

If the players are trying to use it as an attack, make them roll initiative. If they have surprised the enemy (which they probably have), you might have to roll well on the initiative for the monster in order to be able to have them pick up the cube or boot it away ( they need to be higher in the turn order than the PC that threw the cube in order to do something about it next round)

The more monsters they try to catch in its aoe, the more chances there are for their plan to go wrong if you rule it like this.

Worst case scenario, if it's annoying you, make it require a Dex check to aim it where they intend it to go. Give it a high DC if they are trying to throw it onto an uneven surface, like rocks or cobblestones, and on a failure of 5 or more have it fall into a sewer or into lava or some other environmental hazard. 

If you really just want them to not chuck it like a grenade, rule that it has a symbol representing the top that must be facing upright in order to activate. If they try to throw it, hand them a d6 and tell them that unless it rolls a 6 nothing happens as the cube has landed the wrong way. Or that activating it on the wrong side destroys the enchantment, your choice.

This was a fun thought experiment, thanks for your comment!

Quote from Cappuls >>

As a DM, I generally rule that casting Mending on this causes it to regain 1d10 hit points.

You shouldn't. Mending specifically says that casting it on a magical item repairs the item, but does not restore magical properties. So if you break a magic wand in half and cast mending to repair it - great, you now have a perfectly mended mundane stick. Also, mechanically speaking, that is way unbalanced. Mending is cantrip and can be cast endlessly given enough time. The fortress provides a situation where the player can cast endlessly with impunity. I'm sorry, I normally wouldn't tell another DM they shouldn't do something and you should always do what works best for your game, but this is definitely not possible with Mending or the Fortress as written.

What if you have two and you put an inactivated one inside the big one, then shrink the big one and grow the first one inside the other?

I believe it's like the Magnificent Mansion, where it spits out anything inside when it shrinks back down, so getting a second cube to stay inside wouldn't be possible.

Quote from Cappuls >>

As a DM, I generally rule that casting Mending on this causes it to regain 1d10 hit points.

 That's okay if you want to homebrew whatever you like. But it's good to note that that violates the intent of both the mending cantrip and this item.

This is an extremely powerful item, only limited by the fact that it can be destroyed if not properly managed, hence why it take the most powerful spell in existence to heal. 

Mending isn't meant to restore the structure of things or cause them to regain hit points, cantrips generally should never restore hit points. Mendings intent is to restore functionality for something that is very lightly damaged. Anything that is cracking adamantine is likely leaving a bigger scratch than what mending is meant to apply to.

But, again, you can homebrew whatever you like if it makes more sense for your world, story or for fun. 

Quote from MartelBW >>

What if you have two and you put an inactivated one inside the big one, then shrink the big one and grow the first one inside the other?

As the description 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."

If there is something in the fortress, it is not empty, therefore oit cannot be shrunk. 

I have a question which is can you cast the spell  Enlarge/Reduce on this magic item and if you can will the damage increase and if so by how much?