You can read the lengthy lore about the class, but the tl;dr is that the prophets did some seriously messed-up stuff involving blood rituals that “put their bodies into a state of toxic shock from which barely half emerged alive. Those who do are permanently altered – blinded, their lips stained and their own blood tainted for as long as their hearts continue to beat.” Well, isn’t that nice?
Prophets use a combination of pistols and blood magic, which allow them to hex their enemies and steal life from them. They’re not the most pleasant of allies, but at least they’re (nominally) human. If it’s them or the vampires, well, who would you choose to meet in a dark alley?