The Insanity War
This war waged for hundreds of years, in which humans were used as simple tools.
After the barriers fell, Devils, Demons, Fey, Shadow and Aberrants all carved out their own empires across the land, and then waged war against each other in the hopes of claiming it all.
Late into the age, two groups of adventurers appeared, each with incredible talents.
Together, along with a long list of legends, they slowly manipulated the various factions into either fighting each other, or joining them against the Aberrants, which over the years had gained the upper hand. Many deals were struck of which the effects would not be immediately obvious.
Eventually, in a final battle, they faced Gibbeth itself, and closed the gates to the outer realms, but also releasing such strict bonds upon the gods.
Salindurthai, one of the Shadowlords, had constructed an Empire which literally used humans as cattle, raising them and training them until they were of age, and then killing them before raising them again as undead.
The southern tip of the continent was destroyed by a soul-bomb, which a group of adventurers hid within the canals of a city Glasya had been inhabiting.
A cleric Jafaar sacrificed himself to let power out of the Astral Sea, resulting in angels of the gods reincarnating in human form – and of the faithful everywhere being able to access their power once more.