In the days of Atlantis, the Supernal Realms and the Material Realm were in direct contact with one another, but travel between Realms was still difficult and dangerous. Mages still Awakened through an astral journey, not a physical one.
One faction of Atlanteans sought to change that, and create the means to bodily enter the Supernal Realms. This sparked the First Wizard's War, between those who recognized that this idea was dangerous hubris and those who wanted to give it a shot. Hubris won, and after the losers had been cast out of the city the remaining Atlanteans constructed the Celestial Ladder and began to physically travel to the Supernal Realms.
It was a very thin line between visiting the Supernal Realms and invading them. Soon, human mages had taken control of the Supernal and were using their immense powers to warp the Tapestry to their whims. The integrity of the entire universe was in danger. Exiled mages infiltrated the Ladder to battle the tyrants, and as a result of this war the Ladder was destroyed. In its place, the Abyss opened up, creating a wasteland of unreality between Earth and the Supernal Realms.