The following post was originally written by LeeJP in this post on /r/bloodborne, with only minor changes made to the format. This post examines the Old Blood, The Great Ones, and how all of this connects to the moon(s).
While the Great Ones play an integral role to the story and setting of Bloodborne, very little is actually known about them: their motivations, their nature, their origins, so on and so forth. Only a handful are actually encountered (and/or killed) through the course of the game, and a couple of others are mentioned but not directly seen or interacted with. Of those, however, two come to mind before the rest: Mergo and the Moon Presence.
These two Great Ones easily bear the most relevance to the player character's journey as a Hunter bound to the Dream, the former being at the heart of the disastrous and seemingly unending Hunt that has thrown Yharnam into its near post-apocalyptic state, and the latter being the conceiver of the Hunter's Dream and the force both sustaining and compelling the player throughout the journey. It is evident that Mergo is the ultimate target of the player's Hunt, even if this is not actually known for a large portion of the game: once Mergo's Wet Nurse is slain and his cries gradually go silent, the player returns to see the Workshop in flames and Gehrman prepared to expel him from the Dream, Burial Blade in hand.
This begs the question, however, of why.
I swear I'm going somewhere with this, so bear with me. If I get a little rambling, it's because it's been a long day and my brain is a little scrambled.
To flesh out my theory, we have to first look at the nature of the Scourge of Beasts, and the reason why Yharnam has more or less collapsed as a society by the time the player character arrives there. We know that Beasthood is something that lurks deep within mankind as a whole, something intrinsic rather than alien (to draw parallels to Dark Souls, something like the Dark that exists within Humanity). From the descriptions of Beast Roar, the Beast Claws, the Beast Blood Pellet, and the Clawmark Rune respectively:
"The indescribable sound is broadcast with the caster's own vocal cords, which begs the question, what terrible things lurk deep within the frames of men?"
"As flesh is flayed and blood is sprayed, the beast within awakens, and in time, the wielder of this weapon surges with both strength and feverish reverie."
"Ripping apart the flesh of one's enemies and being rained up by their splattering blood invigorates one's sense of beasthood, feeding strength and euphoric feeling alike."
"Although the difference is subtle, Runesmith Caryll describes the 'Beast' as a horrific and unwelcome instinct deep within the hearts of men, while 'Clawmark' is an alluring invitation to accept this very nature."
I would guess that beasthood being an intrinsic property isn't something that is exclusive to Humans, however: consider the dilapidated state of Pthumeru, which was once a proud and thriving civilization, but is now overrun with beasts and undead abominations, and whose only remaining peoples are nearly-feral and decidedly primitive. The Pthumerians were exposed to the Old Blood long before Humanity, given their close relationship with the Great Ones prior to their mass-ascension, and had collapsed long before the first Human expeditions into the Chalice Dungeons. I would think it's fair to assume that Pthumeru fell to the Scourge of Beasts in much the same way that Yharnam did, and that the Old Blood brought out the Beasthood in them just as it did in Humans: those Pthumerians that remain are crazed and degenerate, perhaps an example of what might become of those in Yharnam who don't immediately become beasts. Beasthood seems to be representative of the primal instincts that all life possesses to varying degrees, and that intelligent species such as the Pthumerians and Humans still harbor deep inside. It seems that the only things that prevents a descent into beasthood in those that have imbibed the Old Blood are discipline and force of will, both of which are representative of those things that make us Human.
At any rate, we have a fairly good idea that it is the Old Blood that causes beasthood to express itself dominantly. But what is "the Old Blood" exactly? From our encounters with Great Ones throughout the game, we can clearly see that they don't all have the exact same blood (this is as simple as considering that their blood isn't even a uniform color). And how could they? All Great Ones are decidedly unique, and not all Great Ones share a common origin (some being ascended from other species). Rather, I believe that "the Old Blood" that is referred to isn't simply the blood of Great Ones, but more specifically the blood of Oedon.
Oedon is formless, but according to the descriptions of the Formless Oedon and Oedon Writhe Runes,
"Human or no, the oozing blood is a medium of the highest grade, and the essence of the formless Great One, Oedon."
The blood of ordinary creatures in and of itself has no incredible properties: it is only with the introduction of the Old Blood that it becomes more than mundane. As I see it, the essence of Oedon, a Great One, is what gives the blood of humans (and others) remarkable qualities.
If the Old Blood is, indeed, the blood of Oedon, then it would follow that Oedon is directly responsible for the Scourge of Beasts. This is something that in turn meshes well with the theory that Mergo, another formless Great One, is the child of Oedon: after all, Mergo and the Red Moon are arguably responsible for the collapse of Yharnam. From the Lore Notes in Old Yharnam, the Hypogean Gaol, and Byrgenwerth respectively:
"The red moon hangs low, and beasts rule the streets. Are we left no other choice, than to burn it all to cinders?"
"When the red moon hangs low, the line between man and beast is blurred. And when the Great Ones descend, a womb will be blessed with child."
"The Mensis ritual must be stopped, lest we all become beasts."
The Mensis Ritual beckoned Mergo and the Red Moon, and the appearance of the Red Moon signals a worsening of the Beastly Scourge. Further, a note from the Hunter's Dream:
"To escape this dreadful Hunter's Dream, halt the source of the spreading scourge of beasts, lest the night carry on forever."
And when is the player character allowed to escape the Dream? When they have killed Mergo, in turn ending the Nightmare, the Ritual, and the Red Moon. It would seem that Mergo and the Red Moon are the cause for the spread of the scourge.
While others would suggest that the Red Moon is the Moon Presence, this just doesn't seem to make sense at all to me: after all, the Moon Presence empowers and compels the player character to kill Mergo and end the spread of the scourge, which in turn began with the Ritual of Mensis and the beckoning of Mergo and the Red Moon.
Rather, consider that each time the Moon Presence is referred to, it is referred to as "Pale". This is in contrast to the Red Moon, which is specifically referred to as such. What else is a Red Moon referred to as? A Blood Moon. And what is Blood? The essence of Oedon, who is incidentally "formless".
TLDR: The "Old Blood" that started the Church and turns people into beasts is Oedon's Blood, Oedon is the Red Moon and spreads beasthood, the Moon Presence is the Pale Moon and is Oedon's rival, killing his son and limiting the spread of beasthood by conceiving a Dream that constantly revives Hunters.
Of course, this is just a theory, and a rambling, non-concise one at that. And naturally, there are still many things that are unexplained. For example, Oedon's motives in causing the scourge, and the Moon Presence's reasons for opposing him, among others. It's just what I've found to make the most sense personally, and what I roll with. Feel free to give your own input.