Whoever fights monsters should see to it that in the process he does not become a monster, on account that a bunch of monsters can turn you into a monster by biting you. Vampires, werewolves, zombies. those are the big ones, there are others
The transformation of monster hunters into monsters is actually far more common than most realize. The Registry of the Changed in the great library of Vos-Ulthar contains over 6,000 documented cases, and those are just the ones who lived long enough to be catalogued.
Beyond the classic triumvirate of vampirism, lycanthropy, and zombification (which the esteemed vampire hunter Marienne du Sang rather amusingly termed "The Big Three" shortly before becoming a vampire herself), there are numerous other transformative conditions that can affect monster hunters:
The Hollow Curse: Contracted from slaying too many void wraiths. Your insides slowly become an empty space that hungers for reality itself. The final stages involve your skin becoming a thin membrane barely containing an infinite darkness. The only known cure involves filling yourself with sanctified lead, which is exactly as pleasant as it sounds.
Gargoylification: Not actually from gargoyles (who are quite pleasant once you get to know them), but from the parasitic stone-spirits that sometimes inhabit them. Turns your flesh to living stone over the course of months. The Brotherhood of the Hammer actually seeks this out deliberately - they claim it makes them better hunters.
The Hive-Mind Plague: Spread by the psychic residue left in the corpses of hiveminds. Slowly connects you to every other infected person until you lose your individual identity. The worst part? You remain conscious the entire time, just… shared. The Collective of St. Sebastian contains over 300 former monster hunters, all sharing the same thoughts.
The Butcher's Mark: This one's nasty. Kill enough things that bleed, and their blood starts to recognize you. You become a focal point for blood itself. It seeks you out, tries to drown you. The famous hunter Klaus von Rothstein ended up floating in a sphere of every drop of blood within a mile radius. He's still technically alive in there, they say.
A few practical tips from the Hunter's Black Book (written anonymously by someone who was clearly turning into something awful):
Never hunt the same type of monster more than three times in a row. They start to recognize you on a metaphysical level. Carry blessed silver AND cursed iron. Sometimes you need to fight corruption with corruption. If your reflection starts giving you hunting advice, STOP IMMEDIATELY. The Rite of St. Mercurius can temporarily halt most transformations, but each use cuts your remaining lifespan in half. If you find yourself understanding monster languages without having learned them, it's already too late.
There's also the philosophical question posed by the scholar-hunter Yavina the Bright: "If we accept that all monsters were once something else, then perhaps becoming a monster is simply the universe's way of maintaining equilibrium." She wrote this shortly before her transformation into living stained glass, which honestly just strengthens her point.
The Monastery of the Eternal Vigil actually maintains a special wing for transformed hunters, treating them as honored guests rather than monsters. Their philosophy is that someone who gave their humanity in service of protecting others deserves respect, not silver bullets. Of course, they also keep them in cells lined with every ward known to man, but it's the thought that counts.
Remember: if you start experiencing symptoms of transformation, don't panic. Panic accelerates most transformative conditions. Instead, calmly make your way to the nearest temple of St. Ophelia the Unchanging, or if that's not possible, find the deepest hole you can and jump in. Sometimes the old solutions are the best solutions.
A final note from the Hunter's Black Book: "If you're reading this, I'm probably something terrible now. But I protected people. Remember that. Whatever I am now, I PROTECTED PEOPLE."
