At the first moment the Undead Tribe successfully occupied Luskan, the Temporal Artifact in the Alchemy Hall hurried to devour a large number of Crystal Stones to create a duplicate of itself; however, the Crystal Stones consumed were not from William's small box.
After acquiring that box which appeared small yet held vast space, the Temporal Artifact used the energy from those Crystal Stones to establish a small space teleportation tunnel, using the location of a space-time Elemental Spirit within the Elemental Realm as a transit point, directly connecting to the Divine Kingdom of the God of Time and Space.
Even though the Divine Kingdom of the God of Time and Space is closed, it doesn't mean William cannot enter. Although they finally have some distinctions, they are still as inseparable as some telepathic twins.
In this way, even if the space on Toril's side is unstable, items can be temporarily stored in the Elemental Realm for a while.
