Hi. 
Imagine this:
The PCs visit a city on some backwater Material world. A gloomy dark-ages atmosphere dominates the city and the whole realm wherein the city is situated. That city is overshadowed by a huge, gothic-style temple of a god of order whose will is executed by a merciless high priest and his clergy. Assisted an order of remorseless inquisitors he conducts a hunt on heretics, demon-worshipers and sinners. Whoever confesses of his sins by himself may find absolution by serving in a 'holy war' in a land far away. Whoever is found to be a demon-worshiper is sentenced to death.
Truth is, there's indeed a cult of an Abyssal Lord operating in the city, lead by a demon, growing with the growing oppression and fighting the minions of the high priest. But rarely one of the sods the inquisition catches actually belongs to the cult. There is also good-aligned resistance, both among common residents of the city and the nobility, but they have to fear the temple's influence and power themselves.
Truth is also, that the temples clergy has, over a long time, been corrupted by baatezu. The former high-ups of the temple have been replaced by a cornugon that poses as high priest (maybe casting spells granted by one of the arch devils?) and a host of erinyes, all disguised as humans. And the convicts sent far away to find forgiveness for their 'sins' are actually dragged to the lower planes to fight against the tanar'ri as cheap conscripts in the Blood War.
So far, it sounds quite interesting, at least to me. 
The problem is: Who cares? Would the factions be interested in demons and devils fighting a part of their war on some material world? How could the PCs get involved?
I was thinking about something like a rescue-mission where the PCs have to find and free a planar friend/ally who has been caught by the high priest (maybe because of being a tiefling?) and is soon to be executed. Thereby they could get involved with the various resisting groups, including the demons and fight out what really is going on.
What do you think?
Certainly if a tiefling friend of theirs has fallen afoul of religious zealots (or just peery devils that know a tiefling might be wise enough to expose them), and can get a message out, that's a fine adventure hook.
If any of the PCs are of the church of the temple in question, their planar hierarchy may have noticed the disruption in the local organization and sent them to investigate.
Experience in the Blood War may find them running across one or more of the conscripts and getting the story from them; plenty of PCs have reasons to oppose the machinations of devils.
If any of the PCs or the PCs' allies have ties to the Prime, their homeworld could be the one where this is all happening, so they'd hear from friends and family about the slowly worsening grind back home.
Members of the Harmonium or Fraternity of Order might simply receive orders to the effect of "an ally of ours has requested our services in accordance with standing agreements; you're assigned to investigate and, if possible, rectify." Athar might take the opportunity to not only expose the fraud but offer an entirely different solution to the problem.