I could really need some help fleshing out how much of an impact a small group of Baatezu could have on the late 19th century Earth. Here is the general idea so far (but I´m open for any suggestions):
A small group of powerful devils have arrived on Earth. Among them is the former ruler of Avernus, Zariel. She is weak, the result of Bel draining her power for centuries. She is accompanied by Nergal, master of pestilence. Accompanying these two powerful devils to Earth are a number of lower ranking fiends. A couple of erynies, two cornugons, a handful of barbazu, a few abishai and a dozen or so imps.
Behind the escape to Earth is Moloch. He has a small army, but he does not want to bring it to Earth until he can safely leave again. The exception being a host of mazzikim – incorporeal fiends – that he has gathered. He has no qualms sending them to Earth, where their power to possess mortals may be of great use.
He counts on Nergal to keep Zariel in check (which might be a mistake) and when she is restored to her full might they will return to Hell together to wreak vengeance on their respective enemies (Malagard and Bel). At least that is his plan, it remains to be seen if Zariel will play along with his scheme.
• Where will they go? I´ve been thinking London – the most populous city on Earth and the capitol of the Worlds greatest empire.
• How will they go about seizing power? Who will they charm, possess, or corrupt? Will they go directly for people in high office, or their advisors?
• Zariel needs to feed on thousands of spirits to regain her power. I had an idea: Zariel creates a charity caring for the sick, then gets Nergal to create a cholera epidemic. The kind nurses that brave the epidemic make the sick say an odd prayer to the angel Zariel (unknown to them, but hey, what do you do?) for help. Thus they have forfeited their souls and she can consume them. Do you have any better suggestions?
• What will she do with her power once, or if, she gets it? I imagine she would want to start a war so she can discern who are the mightiest warriors on the planet and recruit them to her cause (I also imagine Nergals wish to spread disease might conflict with such a plan as Zariel does not want potential champions wasting away rather than dying on the field of battle).
Gothic Earth is isolated from the rest of the Multiverse by an entity sometimes referred to as The Red Death (part of a scheme by Apomps to conquer the Prime Material Plane and then the entire Multiverse). Outsiders can enter Earth, but they can never leave, which means most beings with any sense avoids the place. For this reason, Earth has been largely forgotten in the Outer Planes.
A few years ago a group of Earthlings somehow managed the impossible and left their world. These humans have accomplished great feats across the planes. They have also had encounters with forces from Hell – some of which have been violent.
The dethroned prince of Malbolge – Moloch – saw the isolation of Earth as an opportunity. He managed to free Zariel from her imprisonment by Bel on Avernus and sent her to Earth. She is severely weakened, but in time she may regain her strength by feeding on the spirits of this populous world in relative safety (who would want to risk indefinite imprisonment by going after her on Earth?)
This will give Moloch a very powerful ally, and together they might both get revenge on their respective enemies in Hell, as well as reclaiming their thrones. He hopes that Zariel will be able to break the isolation of Earth from within when she has regained her full power, but is not willing to risk imprisonment himself, and will not join her before she succeeds in vanquishing the Red Death).
Zariel and Nergal might meet opposition from humans with some knowledge of fiends, like the Order of Sion (to whom my PCs belong), the Order of St. Peter, and possibly a Solar stranded on Earth (severely weakened after having been cut off from Celestia for a very long time). There is also a fledgling agency in the UK, called Scotland Yard Special Branch X, which deals with the supernatural. In Germany there is a somewhat older government organisation called KDPP, and France also has a similar organisation. There are numerous secret qabals too, fighting the Red Death, that might be mobilised in the battle against the devils.