Asmoeus was the archangel and exarch of one of the mightiest gods, who's name is now lost. Asmodeus used the Ruby Rod to strike down his master. The dying god then turned his realm into the Nine Hells of Baator, and Asmodeus sent his archdevils to wipe …
Nothing is known about Bel's early history, but it is known that he might have ruled over another dominion of hell before Avernus.
Dispater was arguably to first Archdevil to step foot on Dis, and has been ruling it ever since. Other then that, no one knows where the heck he came from.