For anyone wondering what this game is about
The World of Yharnam
Yharnam is a towering, labyrinthine gothic-architectured city, home to both the Healing Church and the Plague. The Church focuses around healing by infusions of blood. Miraculously, blood healing can repair wounds and cure or hinder diseases. Yharnam's whole culture became focused on blood, even to the point of blood-based drinks surpassing alcoholic beverages as the most commonly consumed. Yharnam's populace is apparently viewed as insane or degenerate by the wider world, and the citizens have become extremely distrustful of non-natives as a result.
The Plague
The most common form of the plague appears to be lycanthropy: Humans slowly turning into wolves. Those in the early stages of the plague have lengthened canine teeth, and grow more hair. As they progress, their arms and legs lengthen and the spine hunches over and the face begins to elongate. Eventually they grow fur and their body finishes the change into a full Lycanthrope. Along with the physical transformation comes a mental degradation: At first the victim assumes all people they don't know are hostile and they'll attack on sight. As the alterations progress, the victim becomes more and more savage and hostile, until it can only stand to be around other exactly like it. Sometimes individuals react strangely to the plague; some become unique beasts, and it's rumored that infected Clerics of the Healing Church become great misshapen wolves.
The plague seems to be endemic to Yharnam, requiring periodic hunts for those very far gone. The hunters are both respected and feared: They bring down the worst of the beasts, but are at risk of contamination themselves.
The Healing Church and The Choir
The Healing Church began as an offspring to the Academy teaching, thus their garb is reminiscent of Byrgenwreth student attire. It's point was to worship blood and develop it's healing abilities. Apparently at some point someone brought a vile blood from Byrgenwreth, thus causing a betrayal to Master Willem, this blood became the source of the Plague.
Perhaps the most influential of the organizations, the Healing Church was founded in the wake of the Choir. There were three echelons to the church: black-garbed hunters, white-garbed “doctors,” and the Choir. Though they did not spring from Byrgenwerth directly, they were explorers who found the labyrinth in the wake of that place and discovered the same remnants of the Truth, and used the research done there already to kick off their own research. However, their form of metamorphosis began with Blood Ministrations. In addition, once the church gained enough influence, it shut off all access to the Forbidden Woods (though for fear of the plague of snakes or to restrict anyone else from getting in [principally giving them a monopoly on the Byrgenwerth research], I have no idea).
Blood Ministrations: The art of using blood for healing. It spread fast and hard, and the church used it for everything. The pungent cocktail suggests that it even replaced alcohol in Yharnam for how intoxicating it was. The church rose quickly to incredible prestige for the healing qualities of the blood.
Ashen Blood: Eventually, Old Yharnam was founded (obviously just “Yharnam” at the time), but it was brought low by a strange (the text says “baffling”) illness known as Ashen Blood, which was a worse form of slow/fast poison. The antidote description cites it as being a more severe case, and in the case of Ashen Blood, the antidotes only provided temporary relief. Eventually, the disease (likely spread due to blood ministrations) took over, and those infected turned into beasts. This is why the beasts in Old Yharnam are poisonous, particularly the red-eyed werewolf near the boss, and why the Blood-Starved Beast is coated with poison. After the fall of Old Yharnam, it was closed up, and Central Yharnam was built, along with the Grand Cathedral. This is perhaps the first instance of “hunters” as we know them now being formed.
Black-Garbed Hunters: Though amateur doctors, the black-garbed hunters were an extermination force and a prevention, not a cure. Their goal was to kill those who were infected, turned, and those who may be infected, which made their garb synonymous with fear.
White-Garbed Hunters: Though they were “doctors,” they sought to do the bidding of the Choir, and would apply blood ministrations with the intent of experimentation, rather than healing. Many of the twisted things the church produced were a result of this experimentation. The great giants (with axes and wrecking balls) as well as the grave keepers near them are products of these experiments, and are legendary beasts that would join the church on the hunt.
The Choir: The leaders of the church, The Choir truly believed they could commune with the Great Ones and learn their Truth. They believed they could ascend to the Cosmos (or at least make contact with it, as implied by the gesture), and it came in a sudden and unexpected realization that perhaps while their feet were rooted to the ground, the cosmos was not immeasurably far away, but right above their heads. Together with Ebrietas, they worked tirelessly to understand the Great One’s Truth.
The Orphanage: Children were abducted from the streets of Yharnam and experimented upon, locked away inside this orphanage to become “brilliant minds.” In fact, they became celestial hosts (the big bulbous headed blue guys).One of them even became a Great One (as indicated by the Celestial Emissary boss), or perhaps was already a Great One that guided them. There is quite a hell of a lot of evidence to suggest that Iosefka is either a member of the Choir or the White-Garbed Hunters (didn’t look closely enough at her garb at the time), which is why she believes that transforming victims sent to her is considered “healing them.” However, this does raise implications as to what exactly you are, since you woke up in her clinic.
The Great Ones
The Healing Church has pursued a strange goal for a long, long time. That goal has been hidden from all but the highest of rank among them. To summon or harness the power of the Great Ones, beings not of this world, powerful enough to be seen as gods. They were here, in the labyrinth, long ago. They appear to be much closer than you'd think At Byrgenwerth, men were sent into the labyrinth, and an academy was built to understand the strange discoveries inside, generally called the Truth. It seems the Truth is a terrible thing, slowly breaking the mind of anyone who delves too deeply. Awful things happened at Bygenwerth, and as a result, the church closed off not only the academy, but the entire forest, calling it "forbidden".
Whether or not the Great Ones are gods is entirely academic. The important thing is: They exist. They have an influence on the world. They appear to be ageless, but they can be killed. Their powers are strange things, often involving teleportation or energy or things that the physical world cannot do. Even perceiving the Great Ones properly requires great amounts of Insight--as Master Willem tried to, to his own cost. The strongest of the Great Ones appears to be the Moon Presence, the creator of the Nightmare the Hunter is trapped within. Others are Oedon, Rom, Mergo, Amygdala, Celestial Emissary, Mother Brain, and Ebrietas.
Oedon, who has transcended to the point he doesn't have a physical body but instead is just a voice and influence. He is very powerful and has become sort of "timeless", thus potentially making him the most powerful one, yet the most indirect.
Some items' descriptions tell about Great Ones living in "The Dream" and others living in "The Nightmare" (Which is the case of Amygdala whose function is still to be defined, and Mergo's Wet Nurse). Humans have been able to contact them through the use of Phantasm, little invertebrates found in the Labyrinth.
Pthumeria and the Pthumerians
Pthumeria was an ancient civilization ruled by Queen Yharnam. It was populated by Pthumerians, an ancient race of people who truly understood the Truth of the Great Ones and gained supernatural powers as a result. Queen Yharnam was a medium for the Great Ones, as cited by an item (saying the Choir discovered a medium, though she’s the only likely candidate). Eventually the Pthumerians retreated into a deep labyrinth, with their holy chalices being the only remnants of their ancient rituals to break the seals. Some chalices (such as the ailing Loran chalice) led to other lands, and the Isz Chalice was in particular used in holy communion with the Great Ones.
The Hunter
The Hunter is an outsider to Yharnam, come to get a disease cured through Yharnam's famous Blood Ministration. A doctor treats the Hunter, but only if the Hunter signs a contract to serve the city of Yharnam as a Hunter. As the Hunter goes to sleep on the table, a beast made of blood rises from the floor, but it's driven off by a molotov cocktail thrown by something off-screen. Then the ghoulish little Messengers come out of the floor, and surround the Hunter. The Hunter wakes up in the clinic of the sickroom, and finds a wounded, but still formidable to an unarmed person, lycanthrope in the rooms beyond. And there the journey through the Nightmare begins.