Void

The Void is a catch-all term used by Terrestrial Races to refer to the chaotic, formless & warping metaphysical realm that surrounds their homeworld of Terros. According to the mythologizing of the Order of the Seven Suns, there was a time long ago when there was nothing but the Void, an empty immatereum where the only thing in existence was a single shapeless consciousness. It was that consciousness that first created life, and physical matter, and eventually the world of Terros itself. The creator's terrestrial lifeforms eventually rebelled against it and cast if out from its paradise-world & back into the Void. 
T he creation of Terros has damaged the fabric of the Void itself by partially forcing it to take material form; however, both life and matter itself are antihetical to the Void's being, as the accepted physical laws of reality cannot fully materialize there as normal. As a result the Void manifests as a strange, warped version of conventional reality, where landforms from Terros appear in twisted perverted versions of themselves; red skies, constant ash rain that falls upward from the ground instead of down from the sky, floating mountains that extrude from nothing, great formless deserts made of smooth glass, etc. It is possible to build locations within the Void, although navigating to those locations within its inifinite shifting span requires a mind that is far from sane. It is also possible to destroy a section of the Void as undertaken by Divankul Dul-Sansiska & Metheryl Arcaena; as it is infinite, methematically, you can subtract a value from inifinite space and still have infinite space. There is a barrier that seperates Terros from the Void; passing between the 2 realms is almost impossible, and life from either side can rarely exist on the opposite plain for long without the use of vast amounts of magic & blood. 
The life that populates the Void are called Demons(Demon), offspring of the original creator Vermithrax who share in the warped matter of their home diemnsion and have characteristics that break with conventional laws. They can have impossible powers & physiology that make them far more powerful and dangerous than all terrestrial life. The downside of existence in the Void is that it causes its inhabitants constant mental anguish because the Void's lack of the physical laws of reality--it was never meant to sustain life. No existing mind is capable of maintaining sanity in the face of this, and it fills both the Demons and visting Terrestrials with a powerful and unceasing urge to escape, often leading to conflicts on Terros. 
The Void is said by the Order to be the source of magic itself. Magic represents the ability to break the structure of conventional reality using one's own will; conjuring fireballs, reading minds, etc. It is posited that mages, when casting spells, are drawing upon the power of the Void--indeed, bringing a bit of the Void's illogic into the terrestrial world according to their will.