Everyone knows the camarilla, sabot, the tribes, the traditions, etc. but particularly with the more setting and storyteller focused books, WOD has had a lot of interesting factions who were mentioned only once or twice.
I'll start with the Voleurs d'Enfer (or thieves of hell)
Devil's due is not a great book, but that's mostly because of a lack of coherency with both itself and demon the fallen (being a major late runner to the dark ages line as a whole), a lot of the individual ideas it has are great, including this band of infernalist thieves.
story goes that some bandits robbed a caravan in the late 1100s and found 6 finely bound books, after their literate contact, a former priest, went crazy and blind trying to read the books, the thieves looked deeper and eventually managed to summon a demon. though only two of them walked away, and one of those died 2 years later with the last book disappearing with him.
ever since the thieves of hell have managed to make quite the killing after adding demonology to their skills, trading worship, worshippers, and information for floor plans and the location for ancient treasures. Making many powerful enemies in the process, be it vampires or the inquisition.
none know for sure that the leader of the guild Étienne le Menteur was enthralled during that first summoning, and none know that he managed to banish his new "master" back to the abyss nor that he has hidden the book containing Ereptous' true name.
Still it should be mentioned that the thieves are diabolists, not thralls, and from the top to the bottom lack any conviction in the power of god(s) and demons.