Originally a night hag from Hades, the Hag Countess became one of the chief advisers of Moloch, the former Lord of the Sixth. She counseled him to rebel against Asmodeus during the Reckoning of Hell, and when Moloch was overthrown, Asmodeus installed her in his place.
As a night hag rather than a devil, she enjoyed no support from any other Lord of the Nine.
Asmodeus had only promoted the Hag Countess so that the other Lords of the Nine would not notice his designs. He installed his daughter Glasya as the new Lord of the Sixth. He caused the Hag Countess' body to grow to gargantuan size and rupture, turning the formerly mountainous Malbolge into a realm of decaying flesh.