The Celestial War

By this point, there are essentially 18 planes of existence and the Astral Sea; Baator otherwise known as Hell ruled by Devils, The Abyss which is the endless pit of Demons, the 14 planes created by the Celests, The outlands that exist between the Outer planes, and the Great Elemental Plane ruled by what is left of the Primordials. Conflict grew between The Celests, Demons, Devils, and Primordials, and eventually broke into all out war.

Many perished into nothing in the war, and Baator managed to extend its influence over two other realms, and greatly expanded there power. The Abyss managed to spread its influence into two other planes as well, while yet another was ousted of Celests, but a stalemate between the Devils and Demons. The Celests and Divine consolidated their power and thus maintained absolute power over the 7 Upper Planes, (Arcadia, Mount Celestia, Bytopia, Elysium, The Beastlands, Arborea, and Ysgard) leaving a buffer zone between the planes ruled by devils and Demons. (Mechanus and Limbo)
  The neutral realms that separated the upper and lower Planes became pure expressions of Chaos and Order, and so the true shape of the Outer Planes had formed.
The War within the Elemental Plane, was almost universe shattering, as Demons, Devils, Celestials and Elementals vied for control of this now ripe landscape of power. But what ended up occurring is a great power was unleashed, it is still unknown who released this power, but it split the elemental world, and even cracked through the Astral Sea itself. Massive portions of this endless realm broke away, and a whole new plane of existence was formed, consisting of energy from the Astral Plane, massive portions of the great Elemental Plane, and influenced from the Outer Planes and Inner Planes.