This adventure arc took place at high nonepic and low epic levels (18-22), so it may be of some interest to you.
The party had come to know the archdevil Calandrian through prior dealings through the wizard. Through contract and misunderstanding from the party, they managed to gravely insult one of Calandrian's enemies, Kaziglu, the ill-begotten spawn of a minor demigod. He ruled a layer of Hell adjacent to Calandrian's, and there was bad blood between them.
The party, by inciting his wrath, influenced Kaziglu to attack them in the Material where they had fled. Fearing the result of the destruction of their homes, the party gathered all allies they had, and some they didn't know they had, to help them assault Kaziglu directly. (The gathering of allies took three game sessions; they left no one unasked, even powerful NPCs they had little relationship with. This actually worked from time to time.)
Of course, this was what brought the group to its untimely end: while the party with its powerful allies managed to defeat Kaziglu *in his own home*, as Calandrian sent legions of diobolic soldiers into the now-unruled layer. Unfortunately one cannot so easily kill such a being, and he rematerialized days later. (Calandrian, after the fight: "Dead? No, you cannot kill him so easily. He will surely return, perhaps in a week.")
The group went into semipermanent hiding in the planes (actually a hard-to-reach demiplane purchased from a contact).
