What happens if the barrier of a Cube of Force collide with an Passwall spell? And with a Wall of Fire for a 2nd time?

Question

I am creating an adventure that contains a Cube of Force and various spells mentioned in its description. I want to get the interactions clear so that I can design with them in mind.

The Cube of Force description says that, if the barrier "is targeted by certain spells or comes into contact with certain spell or magic item effects", the item will lose additional charges.

The mechanics of instant effects are pretty clear: if a Horn of Blasting is activated and the thunder area overlaps with any part of the barrier, or if a Disintegrate spell targets the barrier, some charges are lost. Non instant effects are not so clear:

• If a Passwall spell is cast targeting the barrier, 1d6 charges are lost, but does the Passwall create a hole in the barrier? Note: as said in the accepted answer, a Cube of Force is not a valid target of a Passwall spell.

• If there is a preexisting Passwall affecting a wall, and the barrier collides with the hole created by the Passwall. Does the spell cause the Cube of Force to lose charges and/or the barrier simply cover the hole while they overlap?

• If you make the barrier to collide with a Wall of Fire, 1d4 charges are lost. If you take back and then forward, so you collide again with the same Wall of Fire, does the cube lose another d4 charges?.

Answer 1

You will never have to worry about passwall targeting the barrier

Passwall's description states (emphasis mine):

A passage appears at a point of your choice that you can see on a wooden, plaster, or stone surface (such as a wall, a ceiling, or a floor) within range, and lasts for the Duration.

The surface of the Cube of Force creates "a barrier of Invisible force". If the caster cannot see the barrier, typically requiring a spell like See Invisibility, then they cannot target the barrier. But even if they can see it, the barrier isn't wood, plaster, or stone. Thus, it is an invalid target for Passwall as far as a direct cast.

With that in mind, the only thing we need to worry about is the cube's barrier overlapping with an opening, created by Passwall, on a different surface. In that case, Passwall's effect has nothing to do with opening a passage in anything other than the surface it was originally cast on. Thus, we don't have to worry about any effect applying to the cube, aside from the specific description in the cube's text.

The cube loses 1d6 charges because the item's description says it does, but no hole is created in the barrier because the cube didn't say that would happen, and the Passwall spell wasn't cast on the barrier.

Repeatedly colliding with a Wall of Fire will continue to drain charges

The cube's description only states:

The cube loses charges when the barrier is targeted by certain spells or comes into contact with certain spell or magic item effects

The text makes no mention of "The first time the barrier is targeted..." in contrast to many other effects that do use this language. For example Wall of Fire itself, which says:

A creature takes the same damage when it enters the wall for the first time on a turn or ends its turn there.

From this, we can determine that no matter how many times you run into the same Wall of Fire, the cube will continually lose charges, because there is no exception which states that the drain was only meant to be applied one time.