Age of Chaos

After the Age of Attrition, the gods realized they had lost sight of what they created in the first place, life. They made a pact to stay out of the affairs of the natural world, in hopes the species would develop naturally. This drew the attention of other, more wrathful gods. Io created the dragonborn and gave them advantages that made them superior to the already existing races, including adding dragons amongst their armies. Asmodeus signed a blood pact with an entire country of humans to create the teiflings, giving them unnatural demonic power. Between the teiflings and the dragonborn, all other races were slaughtered or made as slaves. Eventually the teiflings and the dragonborn turned on one another and almost wiped out sentient life again.

For their treachery, Asmodeus was banished to the Abyss, where he is unable to leave. Io refused exile and was slain by the other gods, causing his essence to split between the siblings Bahamut and Tiamat.