The Void between universes exists outside the Ether, and crossing it is the only means of reaching other parts of the multiverse. It is an extremely dangerous place, and only a Void Mage can traverse it safely. Reality has no meaning, space does not obey and natural laws, and magic does not function properly.
The Void is filled with otherworldly horrors the likes of which cannot exist within a universe, not even in the Deep Ether, which has strange beings of its own. Void horrors are not friendly or welcoming, and even the most benign of them may accidentally destroy would-be Void travellers.