Going Down Swinging

Last session, the party was almost captured by the devil clad in chains, but the party broke free and held their own. It was a long battle but after a while the party hears a booming voice, telling the party to stop. A bony white devil, emerges from a nearby hallway offering the party a deal. In exchange for helping the party clear their path to their destination, the party need to clear out a nearby portal from demons and white monsters that are roaming about.