Given the Jigsaw Puzzle Plot of Bloodborne, some interpretation and speculation is inevitable when trying to put its Story Breadcrumbs into a semblance of a coherent Back Story. Nevertheless, the following text will attempt to weld together only the canonical in-game sources with as little speculation as necessary for narrative and historical consistency.
Ancient HistoryThe Great Ones Ascend. The Great Ones exist in some manner for an unknown span of time, but many choose to abandon their physical forms and take on formless existences. Those who remain behind are treated as having not "ascended" and either sleep or seem to seek ways to "ascend".
Ancient Pthumeru. The Pthumerians are an ancient humanoid, yet superhuman civilization that serves as the caretakers of the old underground Labyrinth (a.k.a. "the tomb of the gods"), guarding the Great Ones slumbering there. Through this service, the Pthumerians come closer to the eldritch truths of reality, develop the mystical arts of fire and, in particular, of blood, and build great cities like Ihyll, Loran, and Isz.
Blood Moon Rises. It is in Loran that the Scourge of the Beast first appears, transforming people into werewolf-like beasts. The Blood Moon — an aspect of a Great One also manifesting as the Moon Presence — rises, perhaps, for the first timenote . In the chaos, the last Pthumerian Queen, Yharnam, is impregnated by the Great Ones, but her child, an infant Great One named Mergo, does not survive his own birth (at least, not in the Waking World, as his alien consciousness continues to exist in the Dreamlands).
ByrgenwerthFounding of Yharnam. Many centuries (or millennia) after the decline of the Pthumerians, modern humans build a city in the valley above the old Labyrinth and name it in honor of Yharnam. Sometime later, a school known as the Byrgenwerth College is established on the shores of a large lake across the woods west of Yharnam, while on a island far to its north-west, the Cainhurst Castle is built to house human nobility.
Discovery of the Old Blood. It is from Byrgenwerth that a group of young scholars, led by Provost Willem and including his students Laurence, Micolash, and Carryl, gains entry into the Pthumerian Labyrinth below. There, they discover a source of the Old Blood and, by extension, the existence of the Great Ones from whom it originated. Spurred by this discovery, the scholars start researching ways to accelerate humanity's evolution and to help it ascend to the Great Ones' plane of existence.
Caryll Runes. Runesmith Caryll starts transcribing the inhuman whispers of the Great Ones into symbols named after her (although not all Caryll Runes were actually produced by Caryll). It is never confirmed what happens to her afterwards, but she appears to have died or left Byrgenwerth.
Theft of Forbidden Blood. An unnamed scholar betrays Byrgenwerth and brings "forbidden blood"note back to the Cainhurst Castle. Upon imbibing it, Queen Annalise of Cainhurst acquires Complete Immortality, becoming the first of what later become called "Vilebloods", and sends her royal guards out to collect Blood Dregs for her, hoping to conceive a "Child of Blood".
Fishing Hamlet Massacre. The scholars learn of a Great One (Kos) beached near the Fishing Hamlet and dispatch their agents — a skilled craftsman named Gehrman and Lady Maria, an estranged relative of Queen Annalise, — there to retrieve anything useful for their research. The duo slaughters the locals on their way to Kos' resting place, forcibly searching their skulls for eyes-on-the-inside. Once there, they discover the Great One's unborn child (Orphan of Kos) and bring the fetus back to Byrgenworth, although Maria, disgusted by her own actions, throws her Rakuyo down the Hamlet's well on their way back and swears off hunting. At Byrgenwerth, Master Willem dissects the fetus, obtaining an eldritch artifact known as "Cord of the Eye" (a.k.a. "One Third of an Umbilical Cord"). In retaliation, the surviving Fish Kin and Kos place a curse on Gehrman, Maria, and all current and future hunters (metaphorical descendants of the two) to succumb to their own Bloodlust and to be drawn into the Hunter's Nightmare.
The Great Schism. A major ideological rift takes shape among the Byrgenwerth scholars: Master Willem believes that humanity's ascension should be brought about via Insight and "lining their brains with eyes", whereas Laurence contends that it should come through imbibing the Old Blood. Eventually, their contradictions become irreconcilable, and Laurence, Micolash, Gehrman, Maria, and an unknown number of students leave Byrgenwerth, taking the College's source of the Old Blood with them.
Early Days of the ChurchFounding of the Healing Church. In Yharnam, Laurence starts the Healing Church and becomes its First Vicar, erecting the Grand Cathedral north of the city. Unbeknownst to lay people, a secret Research Hall is built above the nave (with the altar hiding an elevator), where Lady Maria carries on with the research into humanity's ascension. At the same time, Micolash independently continues his studies at Yahar'gul, the Unseen Village. His students become known as the School of Mensis, secretly sending hunters and Snatchers out to procure test subjects for their experiments.
Return of the Scourge. The Church advances Blood Ministration as a panacea for all diseases, attracting the sick from all over the world with its miraculous healing powers. In reality, they use it to study the Old Blood, whose biggest unintended side effect is the reemergence of the Scourge of the Beast. When the first Yharnamites transform into beasts, Gehrman establishes his Workshop and becomes the First Hunter. His pupils operate in secret, disposing of transformed beasts under the cover of night to prevent mass panic. They also are used to dispose of potential non-Beast threats in the city, using the Beast threat as a justification to remove anything in their way.
Cainhurst Massacre. Upon learning of the Vilebloods' existence, the Church forms the Executioners: a cadre of fanatics who storm the Cainhurst Castle and slaughter everyone inside, with only their leader Logarius surviving to confront Queen Annalise. Unable to kill her for good, he martyrs himself to seal her away forever. The Executioners' uniforms, particularly Logarius' shawl, become the antecedents of all future Church garbs.
Lady Maria's Suicide. Unable to cope with her guilt over the Fishing Hamlet massacre and the failures of her research on the Brain Fluid, Lady Maria commits suicide, slitting her own throat. Gehrman spirals into depression and creates the Plain Doll in her image.
Birth of the Hunter's Dream. Vicar Laurence and his associates — Gehrman chief among them — procure another Cord of the Eye and beckon the Nameless Moon Presence (a.k.a. "pale moon"). An unknown deal is made with it, and Gehrman is taken from the Waking World to oversee the newly-created Hunter's Dream (where the Doll is given a semblance of life) to guide future Dream-sustained Hunters. Seemingly as part of the deal, Gehrman is to wait for Laurence to find a way to rescue him. In his absence, however, the old Workshop falls apart.
Founding of the League. A group of foreign constables chases a beast to Yharnam and perishes fighting it. Its Sole Survivor, Valtr, devours the beast's corpse, gaining the ability to perceive Vermin in human blood, and starts the League of Confederates to discover and to crush all of it.
Tragedy of Old YharnamAshen Blood. A deadly disease known as "Ashen Blood" spreads through Old Yharnam. After traditional medicine proves ineffective, the Healing Church carries out Blood Ministration on a previously unseen scale. Predictably, this causes scores of Old Yharnamites to transform into beasts, and the Blood Moon rises again. With the Workshop Hunters in disarray, the Oto Workshop is established and sent into Old Yharnam instead. These new hunters soon evolve into "the Powder Kegs", who eschew Gehrman's clandestine ways and instead adopt unsubtle, high-powered weaponry. Still unable to stem the tide of the Scourge, however, the Powder Kegs resort to burning it all to the ground, finally stopping its spread. The charred ruins of Old Yharnam are sealed off from the rest of the city.
Djura and the Dream. During the Old Yharnam outbreak, a hunter named Djura fights alongside the Powder Kegs before being drawn into the Hunter's Dream and guided by Gehrman. After the burning of Old Yharnam, Djura is consumed with guilt and lets Gehrman sever him from the Dream, so he can return to the ruins and care for the beasts who survived the fire. Of his companions, one stays with Djura, while another is drawn into the Hunters Nightmare.
Aftermath of Old Yharnam. Gehrman's old Workshop is formally retired and sealed away, causing many techniques and weapons to be lost. The Oto Workshop and the Powder Kegs are denounced as heretics for their actions by the Healing Church. The aging Vicar Laurence creates and memorizes the Beast's Embrace rune in an attempt to control the Scourge, but instead transforms into the very first Cleric Beast and has to be put down. Nevertheless, his Healing Church continues to grow in his absence, becoming the de facto governing body of Yharnam.
Rise of the Healing ChurchHealing Church Workshop. In need of new hunters, the Church establishes its own Workshop in the Cathedral Ward and appoints Ludwig the Holy Blade as its captain. Ludwig recruits many clerics as hunters, but also arms and trains common Yharnamites. This allows the Church to adopt a new strategy of periodically calling for city-wide Hunts that purge large numbers of beasts at once. Sometime later, an eccentric member of the Church Workshop named Archibald invents several hunter tools based off Darkbeasts' electric powers, while Ludwig acquires the Holy Moonlight Sword and is guided by it on Hunts. While Ludwig eventually succumbs to bloodlust and is drawn into the Hunter's Nightmare, his Workshop continues its service and his Moonlight Sword goes on to inspire copies of it that many Hunters use.
Choir vs. Mensis. With Lady Maria dead and Micolash increasingly uncooperative, the Church forms the Choir to take over Maria's research. Looking for a new source of Old Blood, they also send Tomb Prospectors into the Pthumerian Labyrinth, where they recover the Great Isz Chalice. The Choir uses it to commune with Ebrietas, Daughter of the Cosmos, eventually bringing her (manifestation) to a secret cavern (Altar of Despair) behind the Grand Cathedral. With Ebrietas' knowledge, the Choir sets up a new lab at the Yharnam Orphanage, using orphans as test subjects. The relations between the School of Mensis and the Choir deteriorate as Mensis tortures a Choir member to death, while the Choir sends a hunter named Edgar to infiltrate the School. Mensis' continued experiments draw many more Amygdalae to Yahar'gul, where they come to be worshiped as gods.
Moonside Lake Rituals. At some point after the Great Schism, several important events take place at Byrgenwerth, though it is unclear when exactly and in which order they occur: Rom the Vacuous Spidernote is placed under the lake near Byrgenwerth to hold back the Blood Moon and, more generally, to protect the minds of Yharnamites from the eldritch truths unraveled by the scholars' research. Master Willem seeks to line his brain with eyes — and succeeds in a way that leaves his body a deformed, unintelligible husk in the Waking World; his students blame Rom for this.note The Byrgenwerth Lecture Hall is ripped from the Waking World and transported into the Nightmare. The Byrgenwerth College and the woods around it are declared forbidden ground by the Church.
Father Gascoigne. A cleric from a foreign land comes to Yharnam and trains with the Church Hunters. Gascoigne partners up with a local hunter named Henryk and saves his life, before eventually parting ways with the Church. He marries a local named Violanote and raises two daughters with her.
Eileen the Crow. Another hunter from a foreign land comes to Yharnam and takes the oath of the Hunter of Hunters. At some point, she is drawn into and eventually severed from the Hunter's Dream, but chooses to remain in Yharnam to continue her oath of hunting other hunters.
Fall of the Healing ChurchAlfred the Vileblood Hunter. A man named Alfred becomes a Hunternote , but sometime during his hunts learns of the history of the Executioners and the Vilebloods, in particular that of Logarius. Inspired by the tale, he decides to focus his efforts on trying to find a way to Cainhurst to avenge Logarius and make him a true martyr. Due to lacking an invitation, he is unable to and remains stuck in Yharnam trying to find a way to Cainhurst.
Vanishing of the Hunters. One day, almost all still-active hunters disappear from Yharnam and pass on to the Hunter's Nightmare, drunk with blood and chasing after beasts.note Yharnamites begin to whisper of the "hunters' sin", the Church Workshop is deserted, and within a few years, even most ministers of the Church no longer recall the old hunters.
Mensis Ritual. Micolash and the School of Mensis obtain the Cord of the Eye left behind from Mergo's birth and prepare a ritual to gain an audience with the stillborn Great One. They succeed, and while every one of them dies in the Waking World, the collective stillbirth of their brains brings forth a new, helpless Great One (Brain of Mensis) from the Dreamlands (Nightmare of Mensis). Only Micolash and Edgar's consciousnesses make it to the Dreamlands intact. The Mensis Ritual also has a cataclysmic impact on Yharnam, but its full effects are held back for a while by the continued existence of Rom the Vacuous Spider under the Moonside Lake.
Final Hunt. The Blood Moon rises again, but is held back by Rom. Another outbreak of the Scourge occurs anyway and prompts the current Vicar of the Healing Church, Amelia, to call for another Hunt. Father Gascoigne heeds the call and sets out to hunt again, but succumbs to the Scourge and kills Viola as she tries to bring him to his senses. The Upper Cathedral Ward falls to the beasts, prompting the remaining Choir researchers to evacuate, including Yurie the Last Scholar (who heads for Byrgenwerth) and False Iosefka (who takes with her the Orphan of Kos' Cord of the Eye, which the Choir had previously retrieved from Willem, and heads for Central Yharnam). A sick person from a foreign land stumbles into Iosefka's Clinic and receives Blood Ministration, before being drawn into the Hunter's Dream.