The omniverse is a collection of worlds and universes.  And between those worlds and universes, there is the Void.

    It goes by several names among the circles of cosmologists and those who study related fields of the occult.  The Umbra.  The Horizon.  The Spirit World.  Though the human race is seemingly trivial against such vastness, such is the power of human destiny that their dreams, visions, hopes and aspirations intoxicated a portion of the Void ages ago, causing it to separate, congeal and transform into the Dreaming.  The rest continues to exist as the Void.

    Wormholes, warps and portals will allow travellers to pass through the Void unmolested as they make their way from one realm to the next, but those who wish to explore the Void itself must usually transform wholly or partially into a sort of ectoplasm, the stuff of the spirit world.  This transformation will allow even mortals to survive in the Void indefinitely--removed from such obligations as breathing, eating, excreting waste and regulating body temperature--though it offers no protection from the countless spirits who inhabit the Void, some spirits being benevolent, some neutral and some hostile or monstrous.  The Technocracy has built Voidcrafts for this same purpose, carrying their crews through the Void yet denying the crew members any intimate contact with the Void's spiritual nature...which can be a blessing or a foil, depending on one's viewpoint.  Peculiar and random symptoms have been known to follow direct exposure to the Void, most noticeably in supernaturals; Some paranormal powers or faults are weakened or nullified, while others are exaggerated to embarassing proportions.

    The Void contains everything, yet it itself is nothing.  Dreamspeaker mages contact the Void through vision quests for the same reasons that the Technocracy's Void Engineers fuel their Voidcrafts and set forth: Curiosity.  Searches for answers.  The desire to truly know Creation.  Perhaps even the endless quest for peace, within or without.  And the further one travels from the seat of the Creation, the more nothingness the Void becomes.  The Deep Void is almost utterly hostile to sentient beings; How fitting it was that the Elder Gods were cast into the Deep Void eons ago.

    One who stares into the farthest reaches of the Deep Void would almost believe that there was no chance at all of the Elder Gods crawling back from such endlessness to plague and ultimately consume the living worlds.

   
Almost.
There is no map of the Void--and there never will be--for by its definition it defies the very notions of time and space.  One could sooner chart the entire MIlky Way galaxy than commit the Void to a mere map.
to The Cabal
to the Omniverse.