Aetherial Vortex

Aetherial Vortex is a vortex of energy surrounding the World Engine, in the very middle of the Gods' Realm of the Imperial Religion. Some claim that it's formed by ejection of power that didn't fit even in the infinite Engine, other's claim that it's equivalent to heat and occasional steam output produced by a working steam machine as a side effect.
Whatever it is, the effect is a place that's utterly uninhabitable, while also providing defense for the World Engine from most of potential threats.

Geography

Aetherial Vortex is a place of energies so great, that everything that tries to exist there (including entire planes) it incinerates instantly. There are still some ruined remains of collapsing planes, that can be used to traverse it, but they are still deadly enough to kill even mightiest archdaemons. Merely demigods have a chance. And they will have to face armies of 'eyeless but ever-watchful' guardians of Destiny, making sure that nobody can get too close to the Engine.
It's powers spread further, creating Elemental Wastes.