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?.