I also figured that Belphegor, the Defiler Twinsoul of the Radiant Champion, would make for a very interesting Ambrose Mogens so I came up with a complicated tale of time travel and horror. Basically in the Victorian Era, Ambrose Mogens is Belphigor (the Radiant Champion) but by the mid to late 1890s he is pulled out of his body by his sister who assumes his place. As a Defiler of Transfiguration, she has the special power of being able to appear as any other Fallen while in her true form so she can better masquerade as him.
What does she do with her brother? Well, desperate to finally make him understand her she used Zipacna to use one of her gem's to allow herself to impregnate a woman with a nephilim and then used a dark ritual to place her brother's fallen essence inside that baby shortly before it were born. It took a few tries but she was eventually successful using the assistance of the London Raveners to whom she now owes a great debt.
She then reared this nephilim-brother at one of the places of business controlled by Mogens Institute. I'm thinking inside the Half Moon Hotel so that he was right under Gipontel's nose the whole time. Due to the similarity between her name and her brother's, it was a relatively simple matter to hijack any invocations meant to go to him, but she cut out her son's tongue as a baby just to be sure. This is also a good idea because he can control people with his words.
Belphigor is more confused than most fallen as his memories are all the more buried. He is growing up institutionalised, monitored by Zipacna and the trusted few in sections of the hotel that Gipontel never explored (since the tour convinced him that he saw all he needed to see). His nephilim heritage also pulls him in dark directions as he lacks a normal mortal physiology to fall back on *and* he has no host memories to shroud his own. He'd be completely mad, if not for the fact that his body is human enough.
His twinsoul now acts as his father and is trying to groom him in a particular direction, to become like her, but he is at his core so very different to her and that's starting to show. The poor boy ages slower than mortals, though, as fallen do not age if they have any faith within them. Thus she has had to use a Drain Power spell to remove that from him, costing him mental torment here and there, while he was a baby, or cutting him so that he would be forced to heal it.
She also used a few links from the Devil's Chain, an ancient artefact that prevents a fallen from using their magic and keeps him constantly cuffed, though the cuffs themselves aren't always meeting in the middle. So that's going to be an interesting change.
The other Fallen you will find in the campaign (not including several fallen whose names and summoning rituals are included in scrolls and tomes) include:
Egyptian Priestess - Mythocht (Devil Luciferan).
Ssathasa - Ga Gorib (Devourer Ravener).
Justin Geoffrey (author of the People of the Monolith) - Valac (Slayer Cryptic).
The Black Wind - Riyazgor (Scourge Ravener).
Cthulhu - Belial (Defiler Ravener)
Sleeping Thing Beneath The Australian Deserts - Tiamat (Defiler Ravener)
NOTE: I include the factions of some of these people simply because the Fallen might recall them from the pit and not because the Bloody Tongue God holds any connection to Reconcilers (or indeed, to Devils) or the like.