From D&D Wiki
While Unica had interaction with other planes before the Spell, they now only can access one, the Plane of Shadow. Think of it as the Spell driving the Material Plane into the Plane of Shadow.
A better way to think of it would be a thought experiment. The Material Plane was sort of like a snowglobe, restricted from accessing the other planes surrounding it without magic. However, these were inside yet another globe, which is submerged in the Plane of Shadow, which we could think of as water as well. The Spell was powerful enough to move the globe that is the Material Plane through the globe that was the other planes other than the Plane of Shadow, and is now isolated from the other planes inside that globe protecting them from the Plane of Shadow