Hell is the home of the Demons.
Hell was born of the chaos of the First War. With the ancestors of the Angels victorious, they sought to banish the insurgents as punnishment for their crimes. Broken pieces of Heaven were cobbled together and sent to a sealed dimension to serve as a prison until the end of time. Hell soon broke apart again into several seperate worlds that drifted apart, each with it's own unique brand of desolation and torment. Eventually the lands became inhabitted by an evolution of either the location or those that needed to survive it. Such mutations changed the fallen angels into Demons.
Hell was once completely isolated from the Kingdom of Heaven (aside from a few forces left to maintain order), but eventually technologies were created to bridge the divide and enter Heaven once more.
Hell now exists as several seperate worlds that exist independantly, ruled either by feudal states or total anarchy. Key points are also ruled or maintained by angels to keep a close eye on the demons and supress any future attempts at revolting.