Now, there are monsters in this world, not exactly the most original though. You have your vampires, werewolves, zombies, wights, etc...
Monsters were always at odds with humanity, especially the Church. While monsters were physically stronger and faster, mankind had the numbers and the Church, and the Church had
the Belmonts a family of nobles who more or less dedicated their existence to eradicating monsters in !Europe (! means NOT. In context, Europe with the serial numbers filed off), and they were legendary at it, though no one else hunted monsters, so there's that.
One day, however, Mephistopheles (the demon lawyer, not me) decided to end this family. His invention, the MK.1 Cyber Armor proved to be sufficient, and though he killed not just the men, but the women and the children too, the Cyber Armor was destroyed as well.
Now, the SNAFU level of mankind's situation is lowered by several factors:
First, many monsters were just trying to live a quiet life and actively avoided humanity and continue to do so even now. Werewolves always avoided humans, there are many vampires that live in hiding and try not to participate in vampire society. Even among those vampires that do have armies and power, some are busy inventing better topical anesthetics, so their "Bleed for Alucard" blood donation campaign would gain traction, and see little use for war anyway. Zombies are just meat robots under the vampires' control.
Second is that the nanites that dwell on silver surfaces are highly poisonous to monsters and demons (they can't penetrate armor, though). The noble family and their knowledge were blown up with the rest of their castle, but this one thing is widely known.
Still, there's the werewolf equivalent of the provisional IRA who go after influential people and attempt to infect them and thus turn human society against itself until it collapses. Then, there are the conspiring vampires, the Viking vampires, and the serial-killing vampires like Jack, the DVD-ripper.
So the war still rages within, and peaceful monsters still can't hope to negotiate with humanity, as it remained dead-set on total genocide.
To be able to continue that, humans would obviously try to fill the crater that the
Belmonts left and there must have been some roadblocks (check and balance) in place so that the family would remain the Church's Black Ops and nothing more.
Yet, I'm still struggling. Just what pre-existing medieval organization(s) would have the manpower, infrastructure, and size to be a suitable replacement goldfish? Especially since werewolves can convert the living, and vampires can make the dead into zombie grunts.
Tech-level is late-medieval. Humans only know that silver is poisonous to monsters, but are unaware of the nanites. Some vampires are more advanced and even have electricity, but they stay hidden enough for that to be irrelevant. Demons have advanced tech but can't interfere anymore. The DVD-ripper was a joke name by me.
Since just about every second fantasy does this, the setting is the rough outlines (so no specific people or dates, please) of late-medieval Europe, that's where the monsters are, anything beyond it is irrelevant.