I don't have time for a detailed explanation right now,but this should suffice:
The fall was a dramatic realignment that happened as the greatest champion for good gave in to Agares' temptations and succumbed to her own pride, becoming a great champion for evil. Her sphere was always one of dramatic change, so this change had to be very dramatic. Agares and Mulcarn had been planning to corrupt her for some time to let Mulcarn attack creation, but ultimately the choice to fall was hers. (Even though he was in on it, I believe that the spheres of Ice and Fire still don't get along, and so Agares new closeness to the goddess of fire puts a strain on his relationship with the Mulcarn/Auric Ascended. Bhall is stronger and favored, and the less evil Mulcarn/Auric would feel marginalized, possibly enough to move away from evil.)
The other evil gods "fell" too, but hell didn't exist yet at the time so there was nowhere for there wickedness to draw them.
Bhall is not trapped in hell per se, but because of the compact neither she nor any ofhter god is allowed to enter Creation. She could still visit the vaults of other gods though, at least with their permission.
The force of her fall left Bhall unconscious for most of the Age of Ice, but she is very much awake and active now. While she was in a coma she metamorphosed from an embodiment of holy passion to one of wicked rage, changing from one of the most beautiful goddesses to one of the most ugly.
There are actually 7 hells, one vault per evil god before Bhall fell. Ceridwen has worked to connect these 7 vaults, unifying them into 1 hell with several layers. Hell is a demon factory, designed to drive the souls of the departed downward from the hell they first enter (Mulcarn's was the highest, least evil, most livable hell, but most people skip that and go straight to Mammon's hell) though each successive layer, imparted greater vices along the way, until they reach Agares hell and are corrupted enough to truly be called demons.
Bhall fell straight from her holy heaven into Agares hell, and made part of it her own. Nyx went from being a barren world of the ashes left from when Agares first destroyed his creations to containing regions of utter darkness mixed in with regions of hellfire. The angels that fell with Bhall turned into Balors. In time Bhall may corrupt her old vault and have the connections between it and other heavens broken and replaced with connections to hell, but that hasn't happened yet. Her vault has remained holy, and Brigit might just have a chance to usurp her mistress's godhood and ascend to this heaven before it can be corrupted.