When you're not busy narrowly avoiding death at the claws of eldritch abominations, you might notice there are a lot of other hunters scurrying about the world of Bloodborne. They tend to come in two flavors - kind and helpful, or homicidal and intent on wearing your giblets as earrings. But every once in a while you also hear about a third sort, an eminent group called the Old Hunters, heroes of the past and the focus of Bloodborne's single expansion. They're an important part of Yharnam's history, but with little information to go on, it may be hard to see why. Who the hell are the Old Hunters, and what makes them so special?
WARNING: Spoilers for Bloodborne throughout.
To understand how the Old Hunters came to be, you have to go back several centuries. Long before the first hunter made awkward conversation with an eerie but charming doll, a group of explorers discovered a series of labyrinthine caves under the site of present-day Yharnam, filled with troves of bizarre occult artifacts. They eventually learned that the labyrinth was constructed by a race of mysterious humanoids called the Pthumerians, who gained access to the unknowable "eldritch Truth" by communing with godlike Lovecraftian beings called The Great Ones.
That communion involved drinking the Great Ones' blood, called 'Old Blood', and as luck would have it, the explorers found some for the taking in the labyrinth. Fascinated by what they'd discovered, the explorers founded Byrgenwerth Academy near the site of the caves to study the objects, continue extracting from the caverns, and find a way to commune with the Great Ones themselves.
Various schools within Byrgenwerth had different ideas about how to get in touch with these godly beings (the Provost thought "lin[ing] his brain with eyes in order to elevate his thoughts" was a good approach), but the one that got the most traction was blood ministration, backed by a scholar named Laurence. By providing humans with Old Blood to consume (or the blood of people who'd already consumed the Old Blood via second-hand hemoglobin), he and his disciples created a miracle cure for even the the most fatal maladies. People flocked to Laurence's 'Healing Church' and eventually founded the city of Yharnam, where he and his church continued researching the effects of blood communion in secret.
Unfortunately, blood ministration had one tiny negative side-effect: prolonged exposure to Old Blood turns ordinary humans into bloodthirsty monsters, and the right conditions would unleash an epidemic scourge of beasts on Yharnam. One particularly nasty strain led to the city's oldest district being burned down and abandoned, but the population was so addicted to blood that not even the promise of mutation and almost certain death could curb their habits. That meant the Healing Church needed someone to deal with the beasts, and luckily for them, a fellow named Gehrman came up with a brilliant idea.
Gehrman pitied the people who succumbed to the beastly affliction, and decided he would be the one to put them to rest. Fashioning himself a deadly 'trick' weapon that could transform as he needed, Gehrman hit the streets and slaughtered every beast he came across until the threat was dispersed. Astounded by his abilities, Laurence gave Gehrman a workshop on the church grounds to let him continue building his tools of extermination. Eventually, the First Hunter earned himself a robust pack of disciples to whom he taught the hunter’s craft, which spread throughout Yharnam and eventually into other nations, establishing the first wave of Old Hunters. Ironically, they had to frequently feast on Old Blood in order to achieve the strength necessary to defeat beasts, but their extraordinary endurance made it possible for them to consume large quantities and still remain human (which would surely never cease to be the case).
Not about to let one measly epidemic undermine their mission to contact the Great Ones, Laurence and Gehrman then decided to experiment with a piece of an infant Great One's umbilical cord presumably recovered from the labyrinth. You know, just to see what would happen. It's not entirely certain what they actually did with it - developments late in Bloodborne suggest Gehrman might have eaten it, which creates a whole host of new questions. But the result is a little more certain: their efforts caught the attention of a powerful Great One called the Moon Presence, which led to an "encounter with the pale moon" (translation: the moon got big and red) as it descended from the heavens to greet them. That meeting ended with the birth of the Hunter's Dream, a replica of Gehrman' workshop recreated in the world of dreams, and Gehrman being trapped there as its steward with no means of escape.
That was unfortunate for him, but it worked out pretty well for all the other hunters: from then on, during outbreaks of the beastly scourge, they could make a metaphysical contract that bound their soul to the dream and effectively made them immortal. That new power at his disposal, a hunter and cleric named Ludwig became the first 'official' hunter of the Healing Church - though Gehrman's the true First Hunter, the church apparently got lazy and figured two 'first hunters' would be fine, or they wanted to cover up that embarrassing little snafu with him being sucked into another dimension. In any case, Ludwig picked up where Gehrman left off, building up an even larger following and creating even more elaborate weapons (notably his Holy Blade and cannon-like rifle) and established the second wave of Old Hunters.
Unfortunately, the hunters' hubris eventually caught up with them. Turns out a tolerance for blood consumption isn’t the same as immunity to it, and the hunters began to lose their sense of self, often going on deadly rampages or being transported to the Hunter's Nightmare (a grotesque recreation of Yharnam where those overcome with bloodlust eventually end up) before turning into beasts themselves.
Some practiced personal rituals to keep their minds intact - Father Gascoigne, one of the present-day hunters, gave his family a music box to play that would calm his nerves if he started to turn - but their reliance on blood made it impossible to give the addiction up. And, far from turning into simple beasts, their constant exposure to blood turned them into abominations unlike anything the Healing Church had seen before. To overcome the threat that his own disciples presented, Ludwig created weapons of ever-greater firepower, and began recruiting regular Yharnamites to act as 'hunters' and cannon fodder. But even Ludwig's fate was written in blood, and he eventually succumbed, becoming a horrific monster haunting the Hunter's Nightmare.
With few hunters of their caliber to replace them, the struggle against the beasts became a war of attrition with humanity on the losing end. The Hunter's Dream was at least helpful in the struggle, refurbishing dead hunters and sending them back out into the field… but only for a little while. That's where the final, most recent group of Old Hunters come in: though hunters sign a contract that returns them to the dream every time they die, it only lasts for one hunt; once it ends they're sent back out into the world as mere mortals. However, the thirst for blood never disappears, so like soldiers returning from war, most struggle to find a new place in society. Some manage to occupy themselves with new purpose (Eileen the Crow hunts crazed hunters, Djura protects the monsters of Old Yharnam, Alfred tries comically hard to get an invite to a party so he can kill the host), but many, like Gascoigne and his partner Henryk, meet the same fate as all the Old Hunters who came before.
Eventually, the horrible fate of the Old Hunters became a foregone conclusion, and the lesson everyone should have learned was forgotten: they could withstand the effects of the Old Blood where others couldn't, but they didn't treat the process with the appropriate level of humility and care, and they paid for it. Tragic as it is, it's even worse for the people left behind, who have to deal with the fallout from the hunters' 'help'. By the time you show up, the city is being torn apart by beasts and hunters alike, and you have to maintain your sanity long enough to clean up after the Old Hunters' good intentions.