The Shattered World
The dark rise of Kros The Shadow Wight had caught everyone by surprise, not the least of which the gods themselves. Before any of them realized the gravity of the threat, The Unmaker had not only slain several of the more complacent gods, but it had consumed their divine essence, absorbing their power and becoming far more powerful itself.
It took an unprecedented alliance of all the powers in the multiverse, from all alignments, putting aside previous grievances and even full-blown wars, to put together a force capable of challenging Kros. It took time to put such a force together, though, and further time for them to find and finally confront The Shadow Wight; in that time, more gods were slain, their power consumed.
The final battle took place in Kros’ lair deep within the Underdark. A massive army drew the bulk of Kros’ forces out to a confrontation on the surface, while a smaller force infiltrated The Unmaker’s inner sanctum to face Kros itself.
What happened in that battle in The Shadow Wight’s lair is unknown, as no one survived the battle. What is known is that the fight ended in a conflagration of dark, primal energies, both within the lair and within the battle raging on the surface — both armies were completely consumed, body and soul, as well as the infiltration force that had faced Kros itself. Even the entire lair was obliterated.
The immediate aftermath of the slaying of Kros was almost more destructive that its rise to power. All across the multiverse, eruptions of the same dark energy burst out randomly, explosions of pure unmaking, anti-creation, that erased everything they touched from existence completely. Creatures unlucky enough to be caught in the eruptions were obliterated body and soul. No plane was spared, not even the domains of the remaining deities; a couple of deities themselves were actually erased in this event, which came to be known as The Unmaking.
The world itself, the mortal plane, the realm within which the battle itself was waged, was hit the hardest; the very geography of the world was irreversibly altered, and entire nations were consumed. Even the fabric between the planes themselves was not immune to these eruptions, with worldfalls and planar bleeds suddenly becoming fairly commonplace occurrences, where previously they had been almost completely unheard-of.
The worst of it, however, was the sudden discovery of the Far Realm, the nothingness beyond the multiverse. Scholars have speculated that particularly strong eruptions of anti-creation erased not merely the fabric between planes, but the multiverse itself; others have pointed out, however, that this fails to explain the existence of the multitude of new, terrifying, and completely alien creatures that have crossed over through the new rifts into the multiverse, most arriving in the world, but some appearing in other planes. The most likely explanation, therefore, is that the fabric between the planes of the multiverse and the Far Realm is simply much stronger than that between the various planes of the multiverse, and that especially powerful anti-creation bursts were required to breach it.