Seven Kingdoms: Seowyn's Crossing
Asmodeus was a powerful Exarch of a god whose name is not now remembered by mortal kind. As commander of his god’s angelic host, Asmodeus wielded considerable power and was considered a hero of the Dawn War. During the confusion of the War’s mid-period, he slew his god and absorbed the divine spark of godhood, becoming a god himself. Horrified by this act of deicide, Asmodeus and his followers were sealed within the Mourned God’s realm in the Astral Sea. Now rechristened Hell, this realm became Asmodeus’ realm and his prison. He and his followers became Devils, twisted reflections of what they had once been.
Some theorize that Asmodeus was corrupted by the seed of evil that the Nameless planted in the Elemental Chaos. This theory suggests that Asmodeus encountered either the Nameless itself or the seed during a mission in the Chaos. Innocent of its nature, he touched pure evil and became wholly corrupted.
The other, more popular theory is that Asmodeus rankled at his position of serviude, believing that his god remained behind in relative safety while he ventured forth to battle. Asmodeus esented that his god remained safe while he was always in danger of destruction. Finally, his ambition and resentment overcame him, and he took action.
Since he cannot leave his realm, Asmodeus sends forth his devils to tempt and vex mortals, especially humankind. This causes the theory that Asmodeus was a human god, probably a member of The Twelve. Whether this means that they were once The Thirteen or that another god joined to make the Twelve whole again is a matter of some religious debate.