"The veil between worlds is an impassable expanse. Such an impenetrable space, that even the beings hailed as gods for their unimaginable power in face of those weaker, cannot breach this barrier their domains. The Netherworld, The Celestial Plane, The Warp, and the Void. Each a plane governed by the pantheons who were strong enough to wrestle the entirety of their world under their control. However, even with entire worlds at hand, is it not enough. The ancient gods from beyond all seek but one thing, and one thing only. Souls. For souls is the medium that can pass between each plane. It is the currency of the divine.
The one plane sought more than any other. A place abundant with life, incomparable to the domains of gods. For which world is more plentiful than the one at the very core of all these planes. Free from any god or the like. A place of great untapped treasure. Of warring humans, evolving civilizations, and much, much more. Of unbridled life. Of unclaimed souls. The Core World is what all gods seek. It is the key.
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However, as impassable the veil may be, these pantheons have had eons to find a way. And a way, did they find. Each of the four ancient pantheons has its own way of influencing beyond their realms. Whether it be the Infernal Lords, communing with Warlocks and granting them their demonic souls to travel through the veil, or the Gods Oblivion, corrupting the minds of weak men through whispers of the void. Little by little, the ancient gods scheme. A great plan unfolding, time and time again. For an event inevitable to come. When blood rains and the earth tears itself asunder - souls flow free once again. For when the gods do break through the veil. That is the time, that the world becomes hailed by yet another, catastrophic age of blood and death - An age, of The Old Gods' Descent."
— Redacted Records of Arcanum; By The Archmagus Heretic, 3rd Archmagus of Arcanum Approx 500 A.D, Archibald Zelpyrous.