World of Darkness
Throw in Demons, Werewolves, Mages, Wraiths, and Vampires ... and I guess a human faction as well (or just have humans as your general NPCs, meatshield troops, cannonfodder, food, and playthings)
As my parenthesis states ... just have all factions own humans (as slaves or second class citizens, or simply unaware henchmen), with the Champions being the particular monster of choice. Although I would Imagine the "Mage" faction would be the closest you would get to having a full human faction.
Although ... I suppose you could try to have a Witch-Hunter/Demon-hunter/Monster-hunter faction, with extremely weak soldiers (although perhaps slightly stronger than the "meatsheilds" of the other factions) with weaker champions as well.
Then each NPC champion can be either human, mage, vampire, werewolf, demon, or wraith ... and have NPCs hostile to any faction that doesn't resemble them (aka demon NPCs will be always hostile to non demon factions).
At least at first, until considerable subterfuge mechanics are introduced.
Further-more ... I don't see all demons working together ... so I'll jot down my ideas for Factions
Justicars ( Good Demons and Most Mages)/ Good Demons + Technocrats
Renegades (Chaotic Mages and Most Wraiths)/ Nephandi + Wraiths
Armageddonites (Most/Evil Demons and Evil Wraiths)/ Demons, Pentex, and Black Spiral Dancers
Son of Man (Traditional Werewolves and Human Mercenaries)
Keepers (Most/Traditional Vampires, a few Worldly Demons, and Human Slaves/Mercenaries)
Luna's Bounty (Naturalist Werewolves, Shape-shifters, and Druids)/ Werewolves + Most Shapeshifters + elements of Council of Nine
Sixth Chapter (All Humans-> Hunters, Zealots, Businessmen ... all lead by one very cunning/deceptive Vampire)
Forgotten Ones (Savage Vampires, Ghouls, and aberrations/rebels/experiments)