Veil magic

The planes of existence are seperate from each other and yet they overlap. Travellers who have traversed the planes have recounted how certain features on one plane may be found at the exact same spot on another, however warped said feature or site may be. Wizards attempting planar travel describe a sensation of being "whisked through" or "funneled" to their plane of destination. And yet each plane is confined to itself. It is as if it was contained in a bubble, disregarding the sense of three-dimensional space. Using the bubble analogy one could also say that there is a film protecting the planes, a membrane, a layer. And like the surface of water or a bubble is under tension, so is this layer protecting each plane.

Where there is tension, there is energy.

The scholars and scientists of The Arkhenith Empire pursued this line of thought and attempted to tap into this energy source, which then ultimately led to the calamity and the extinction of their empire before The Fall. After decades of research they developed massive magical constructs, whole sites, in order to extract power and energy from the surface tension of the plane veil. A new branch of magical sciences had developed. Veil magic.

The Arkhenith wizards were never able to manipulate such a powerful force by themselves as the veil between the planes can be seen as fundamental as a universal law of physics. But they nevertheless succeeded in harnessing the tension energy though their magical machine-sites.

In their hubris they disregarded the possible dangers of tampering with such primal forces. They thinned out the veil which allowed powerful denizens of the abyssal plane to pour onto the material plane. Once pierced, the veil could be torn at several places.

The rest is history.

Only over a century later the Gods known as The Sacrifices, The Wanderer among them, were able to restore strength to the veil by collectively giving their essences and perishing in the process. Should the veil be weakened on such a large scale again, nothing short of divine intervention would mend it.

All records or instructions of how the Arkhenith succeeded at veil magic were destroyed along with the harnessing sites. There the veil was thinnest and the gleeful destruction of the demons was the most unhinged and the most prevalent. But it is said that a few of these facilities might still be intact and may be found, hidden deep underground or in impossibly remote locations. What powers might they hold?!