The Blood-Starved Beast resides in the Church of the Good Chalice, down in Old Yharnam.
The Blood-Starved Beast is a remnant of the fate that befell Old Yharnam. Its body is made up of old sinew and muscle, any skin is tightly stretched. Affixed to its scalp is a long fleshy looking cloth like substance that appears to have been burned on, as it travels along with the Beast's movements even when it jerks its head violently. At first glance, this cloth resembles the head coverings that the lesser female beast patients of Old Yharnam wear. This resemblance may indicate that the Blood-Starved Beast was once female.
The boss arena that the Blood-Starved Beast fights in appears to be the remains of a cathedral, and its lamp name is ‘Church of the Good Chalice’. The Beast stands at the altar looking towards the Chalice that sits atop it. Gehrman tells the player that the Holy Chalices, retrieved from the Chalice Dungeons beneath Yharnam were once worshipped in Old Yharnam, known as the valley hamlet. From this we can gather that the scholars of Byrgenwerth, who discovered the Chalices, brought them to the people of Old Yharnam, which at the time, was a small settlement without a Church (as the word hamlet implies). The people of Yharnam are notoriously distrustful of outsiders and perhaps they did not immediately take to the use of blood, it is unclear but it seems unlikely that the simple farmers of Old Yharnam would take to the consumption of blood unless it somehow became a necessity.
The spread of the Ashen Blood illness would eventually take root in Old Yharnam. The effects of Ashen Blood were incurable, and death could only be delayed by the consumption of small medicinal tablets. Ashen Blood gave the appearance of a poison, and when this poison builds up in the body, it is able to be spread to others, as can be seen by the player’s interactions with the scourge patients, and the Blood-Starved Beast.
The Antidote description describes the Ashen Blood sickness as baffling, indicating that the people of Old Yharnam had no idea where it came from. It is well known that the blood found by the Healing Church had healing properties, and it makes complete sense that the people of Old Yharnam would therefore turn to the Healing Church in their hour of need to heal them of the sickness. So began the common acceptance of blood usage by the populace of Old Yharnam. The receptacles the Church used to deliver its Old Blood to the citizens were the Holy Chalices mentioned earlier. It is evident from Gehrman's comment that an organised religion began to circulate around these chalices, and the use of blood. In other words, the Church had its first converts.
But in time, the use of blood began to have adverse effects on the population, a prognosis the Healing Church, and its founding scholars certainly knew about beforehand, since it was the cause of the schism within Byrgenwerth that would eventually go on to cause the founding of the Healing Church itself by Laurence and his associates. Use of the Old Blood began turning the people of Old Yharnam into beasts. The ‘new’ disease was known as the ‘scourge of the beast’. Those undergoing the transformation would take on more beastlike qualities, their senses becoming heightened, their strength and stamina increasing, and their body undergoing radical transformation. Those with the illness became known as ‘scourge victims’. Early attempts to cure the illness involved bloodletting, which can be seen in the description of the Bloodletter weapon. Blood was expelled from the body, in an attempt to remove the perceived taint. We can also see this in the Old Yharnam cathedral the player must travel through prior to reaching the Blood-Starved Beast. In this building a crucified beast can be seen hanging as the centerpiece before the altar. The beast’s wrists appear to have been slit, and its blood pools at its feet. This act of crucifying and letting of blood also appears to be symbolized in the Impurity rune.
Evidently, the letting of blood did not reverse the effects of the scourge. The Bloodletter weapon description strongly indicates that Brador, one of the earliest hunters, was misguided in his belief that the expelling of tainted blood would have any beneficial effect:
“This is the only effective means of expelling tainted blood, or
so Brador, isolated in his cell, continued to believe.”
So to counteract this, the Healing Church and its hunters (known as doctors) would, under the cover of darkness, engage in the removal of beast patients who were too far gone. Eventually the spread of the scourge, and the size and ferocity of the beasts became more than the Church was able to handle. Drastic measures were decided upon, and the entirety of Old Yharnam was burned to the ground on a moonlit night. The town was left forsaken and abandoned. Any and all contact with its citizens was securely barred and hidden from the population of the Cathedral Ward in what would become known as Yharnam. Survivors of the burning who attempted to leave the town were hunted and killed without mercy.
Because of the Blood-Starved Beast’s position at the altar in the Church of the Good Chalice, and its unusual size and strength for the area we can extrapolate that the beast was possibly once a leader of the organised religion in the valley hamlet of Old Yharnam. Like Vicar Amelia, the biggest and most powerful beasts are born of those people who have imbibed the most blood. As a leader of the Church, the Blood-Starved Beast would have had access to more blood than the average Yharnamite. Its appearance is very similar, if not identical, to that of the crucified beast hanging in the Cathedral building on the way to the Blood-Starved Beast boss fight. This indicates that the Blood-Starved Beast has received the treatment of bloodletting to try and counteract the scourge. The fight indicates that whoever the Blood-Starved Beast was, they were once infected with the Ashen Blood illness, as the longer the battle goes the more toxic being in the presence of the Beast becomes. It may be that they were one of the first converts to the Healing Church and its blood ministration, but this is speculation.
However we look at it, the Blood-Starved Beast is primarily a victim, of the Ashen Blood illness and of the Healing Church and its ‘scourge’. Its location in the world of Bloodborne indicates that it, like the other citizens of Old Yharnam, simply wants to be left alone, and that it is fiercely protective of the Chalice that once offered it and the people of Old Yharnam hope.