Lawful Evil God of Pride, Fire, & Tyranny
Asmodeus is the evil god of tyranny, fire, and pride. He rules the Nine Hells with an iron fist and a silver tongue. Aside from devils, evil creatures and people desiring power pay him homage. Many warlocks have made bargain with and are drawn to his dark cults. His rules are strict and his punishments harsh. His tenants are to seek power over others, to repay evil with further evil (an eye for an eye), to exploit kindness for personal gain, and to show no compassion for the weak and downtrodden. All done in the most legal possible manner of course, and never overtly. He is also worshiped as a Member of the Dark Six, as god of one of the seven deadly sins, pride.
It is said that Asmodeus was once an angel of a god of kindness and mercy, known only as He Who Was.
During the Dawn War when the gods and primordials battled over creation, Asmodeus was a general in the army of He Who Was. After showing great loyalty and aptitude during the war he was given a task to guard the abyss. Something happened during his time there and afterward he had gone back to Baator the realm of his god and slew him, stealing the power of He Who Was for his own and striking all records of him from history. The Other gods saw this and as punishment bound him to the realm of Baator, thus creating the Nine Hells. All those who aided Asmodeus became devils and he their lord.
Whether this is true or not, mortals may never know.