The Moon Presence is a nameless Great One referred to as Paleblood. It is a being synonymous with the moon and the likely source of the Hunter's Dream.
The Moon Presence is the source of the Scourge of the Beast
"As we know, the Blood Moon rises whenever the line between man and beast is blurred. When humans succumb to the Scourge of the Beast, the Blood Moon rises… or perhaps it's the other way around. Perhaps it's that the Blood Moon rises, causing those infected with the Scourge to transform into Beasts. At the beginning of the game, we find a note in the back of the Hunter's Dream, presumably left for the Hunter: "To escape this dreadful Hunter's Dream, halt the source of the spreading scourge of beasts, lest the night carry on forever." Now recall Gehrman's words to the Hunter after the defeat of Father Gascoigne: "The moon is close. It will be a long hunt tonight." The Blood Moon, or rather the Moon Presence, is the source of the Scourge."
Which would mean that Hidetaki Miyazaki was inspired from popular culture to do with werewolves and their transformation at the moon.
Location: Outside Hypogean Gaol. Leave through the front door. This note is on top of the stairs leading to the area where the Yahar'gul set can be found:
"Behold! A Paleblood sky!"
Location: Found on the right side outside of the cell you wake up in after being captured by a snatcher:
"Madmen toil surreptitiously in rituals to beckon the moon. Uncover their secrets."
Lecture Building note:
Location: In one the rooms at the end of the hallway on the second floor of the Lecture Building:
"The nameless moon presence beckoned by Laurence and his associates. Paleblood."
Byrgenwerth Lore Note:
Location: On a Bookshelf on the same floor as the Lunarium Key door:
"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.
Old Yharnam Lore Note:
Location: Up the stairs from the Bell Chime Maiden Spawn, near the Charred Hunter Set:
"The red moon hangs low, and beasts rule the streets. Are we left no other choice, than to burn it all to cinders?"
Hunter's Dream Lore Note:
Location: Inside the house, behind the Memory Altar:
"To escape this dreadful Hunter's Dream, halt the source of the spreading scourge of beasts, lest the night carry on forever."
Central Yharnam Lore Notes:
Location: Immediately upon waking in Iosefka's Clinic, on a chair to the character's right:
"Handwritten scrawl: Seek paleblood to transcend the hunt."
Location: Within the Dark House near the gate shortcut to Central Yharnam lamp. First accessed from the Great Bridge. Note is on a first floor shelf near the enemy in the wheelchair:
"When The Hunt began, the Healing Church left us, blocking the great bridge to Cathedral Ward, as Old Yharnam burned to the ground that moonlit night."
Opening Cutscene dialogue
“Oh yes… Paleblood. Heh. Well you've come to the right place.
Yharnam is the home of blood ministration, you need only unravel its mystery. But where's an outsider like yourself to begin? Easy, with a bit of Yharnam blood of your own.
But first… you need a contract.
Good, all signed and sealed. Now let's begin the transfusion. Oh don't you worry, whatever happens you may think it all a mere bad dream.”
Upon speaking with him (Second time):
"Paleblood, you say? Hmm… Never heard of it. But if it's blood you're interested in, you should try the Healing Church. The church controls all knowledge on blood ministration, and all varieties of blood. Across the valley to the East of Yharnam lies the town of the Healing Church, known as the Cathedral Ward. And deep within the Cathedral Ward is the old grand cathedral. …the birthplace of the Healing Church's special blood, or so they say. *coughs* Yharnamites don't share much with outsiders. Normally, they wouldn't let you near this place, but… The hunt is on tonight. This might be your chance…"
One Third Umbilical Cords
One of the heirlooms used to contact the Great Ones,
originating in the child of the Vilebloods.
Long ago, in an encounter with the Great Ones,
a contract was established, establishing the hunters
and the Hunter's Dream.
A great relic, also known as the Cord of the Eye. Every
infant Great One has this precursor to the umbilical cord.
Every Great One loses its child, and then yearns for a
surrogate. The Third Umbilical Cord precipitated the
encounter with the pale moon, which beckoned the hunters
and conceived the hunter's dream
Use to gain Insight and, so they say, eyes on the inside,
although no one remembers what that truly entails.
After killing Father Gascoigne:
"The moon is close. It will be a long hunt tonight.”
"This was once a safe haven for hunters. A workshop where hunters used blood to enhance their weapons and flesh. We don't have as many tools as we once did, but… You're welcome to use whatever you find. (Whispers) …Even the doll, should it please you…"
Outside his house after defeating Rom, The Vacuous Spider:
"Oh, Laurence… Master Willem… Somebody help me… Unshackle me please, anybody… I've had enough of this dream… The night blocks all sight… Oh, somebody, please… "
After killing Mergo's Wet Nurse:
"Good Hunter, you've done well, the night is near its end. Now I will show you mercy. You will die, forget the dream, and awake under the morning sun. You will be freed from this terrible Hunter's Dream."
If refused his request or attacked him at the Great Tree:
"Dear, oh dear. What was it? The Hunt? The Blood? Or the horrible dream? It always comes down to the Hunters' helper after all these sort of messes. Tonight, Gehrman joins the hunt."
If killed by Gehrman:
"You must accept your death, be freed from the night."
"The night, and the dream, were long…"
Plain Doll dialogue:
Talk during beginning:
"Hello, good hunter. I am a doll, here in this dream to look after you. Honorable hunter, pursue the echoes of blood, and I will channel them into your strength. You will hunt beasts… and I will be here for you, to embolden your sickly spirit."
"Did you speak with Gehrman? He was a hunter long, long ago, but now serves only to advise them. He is obscure, unseen in the dreaming world. Still, he stays here, in this dream… …such is his purpose…"
"Ahh, the little ones, inhabitants of the dream… They find hunters like yourself, worship, and serve them. Speak words, they do not, but still, aren't they sweet?"
"O Flora, of the moon, of the dream. O little ones, O fleeting will of the ancients… Let the hunter be safe, let him(her) find comfort. And let this dream, his(her) captor… foretell a pleasant awakening… be, one day, a fond, distant memory…"
Retired Hunter Djura’s dialogue:
When the player chooses to spare the beasts:
"Yes, very good. I no longer dream, but I was once a hunter, too."
"You have the whole night to dream. Make the best of it."
Upon killing you:
"I should think you still have dreams? Well, next time you dream, give some thought to the hunt, and its purpose."
Micolash, Host of the Nightmare dialogue:
"No, we shall not abandon the dream."
Old Hunter Bone
The bone of an old hunter whose name is lost.
It is said that he was an apprentice to old Gehrman, and a
practicioner of the art of Quickening, a technique particular
to the first hunters.
It is most appropriate that hunters, carriers of the torch who
are sustained by the dream, would tease an old art from his
Wood-carved mask of Eileen the Crow, hunter of hunters.
The beak contains incense to mask scents of blood and
Hunters of hunters dress as crows to suggest sky burial.
The first hunter of hunters came from a foreign land, and
gave the dead a virtuous native funeral ritual, rather than
impose a blasphemous Yharnam burial service upon them,
with the hope that former compatriots might be returned to
the skies, and find rest in a hunter's dream.
"Even in this darkest of nights, I see…the moonlight…"
Annalise, Queen of the Vilebloods dialogue:
“Visitor… Moon-scented hunter…”
Imposter Iosefka dialogue:
“Ah, moonlit scents….How did you worm your way in here?”
"Oh, my, what a queer scent… But I'd take it over the stench of blood and beasts any day."
Chapel Samaritan dialogue:
“…Hmm? Oh… you must be… a hunter.
Very sorry, the incense must've masked your scent.”
Young Girl dialogue:
"Who… are you?
I don’t know your voice, but I know that smell… Are you a hunter?"
A secret symbol left by Caryll, runesmith of Byrgenwerth.
A transcription of "moon," as spoken by the Great Ones
inhabiting the nightmare.
Gain more Blood Echoes.
The Great Ones that inhabit the nightmare are
sympathetic in spirit, and often answer when called upon.