Lore
Deep underneath Yharnam exists ruins thought to be an ancient resting place for something other than humans. It is rumored that these ruins change their shape depending upon the hunter. Guarding these ruins are ancient guard dogs, beings with stone-like skin and covered in flames.
Also protecting these ruins are hunters. They are born and bred in the ruins, existing on the flesh and blood of the dead. Their skin is pale, and their eyes are black, searching for those who trespass.
The last known inhabitants of these ruins are the ritual masters. Hunters believe that they pray for the resting through a unique ritual. Their appearance and demeanor are horrifying to those that do not know them, unfounded or not. Regardless, they are known to perform a dark and ancient ceremony that few live to talk about.
Theories
Further Lore Explanation
The "superhuman" Pthumerians built the dungeons to house slumbering Great Ones, and in so doing came to "unlock the wisdom of the Eldritch Truth." This gained them access to a myriad of abilities, among them was a thorough understanding of how to manipulate fire. Yharnamites, since the tragedy of Old Yharnam, recognize fire as a purifying force and the Pthumerians used fire to make themselves eternal "in a ceremony of flame that cremated body and soul", in the case of the Keepers and possibly the Watchdogs. It was also used for combat purposes as evidenced by the Rite Keepers, Shadows of Yharnam, several Loran enemies and Pthumerian Elders. They also became intimated with Arcane powers, and perfected the process which creates Celestials long before present day Yharnam with the Choir, as evidenced by their presence in the Isz labyrinth.
Based on what the ring of Betrothal tells us, Pthumerians could presumably be married in the time of the Great Ones, but whether this was exclusive to Pthumerian monarchs(like Queen Yharnam) or if it was available to everyone isn't clear. The ring's description in conjunction with the Bastard of Loran would seem to suggest that Pthumerian infants born out of wedlock were cursed, and the Great Ones weren't too fond of that. So this may have contributed to Loran being "swallowed by the sands", in addition to the beast plague they're said to have suffered from.
At some point in Pthumeru's timeline, the Great Ones mostly seem to have ascended beyond the physical realm. Probably to the Nightmare/Cosmos(fairly sure these are interchangeable terms), leaving behind Ebrietas. Whether this was for a reason or simply negligence is unclear, but I'd imagine it's the former given the rampant notion of Great Ones desiring children.
At some point, presumably late in Pthumeru's timeline, the Pthumerians become pompous enough to elect themselves a monarch, traditionally a woman, one of which was referred to as Ihyll and had a portion of the labyrinth named after her. I'm assuming there's a period of several hundred years or even a millenium during which the Pthumerians become increasingly more… civilized, possibly as a result of contact with the outside world. There is evidence of Pthumerians having interacted with early knights of Cainhurst; we find statues from the labyrinth in Cainhurst, and we find statues from Cainhurst as well as old Cainhurst armor sets in the labyrinth. I find it possible, even probable that they built civilizations on land(one of which used to be in the forbidden woods) and interacted with humans in some form, possibly resulting in their knowledge of gunpowder and other more "modern" day weaponry such as the cannons we find littered about the dungeons.
Presumably the most recent Queen of Pthumeru, Yharnam, seems to have consorted with a Great One(possibly Oedon) to sire what is likely the original "child of Blood", Mergo. A complication of some kind seems to have prevented Mergo's birth and separated her from Yharnam. And I'd suggest, as my own speculation, that this is because Yharnam and Oedon may have consorted outside of the aforementioned wedlock. Maybe this pissed off other Great Ones, and so they took Mergo into the Nightmare and cursed Yharnam's blood? Either way, the Pthumerian "civilization" would seem to have completely fallen by the time we reach it. There's a myriad of beasts and other creatures, a few descendants, an elder and the Queen herself locked up at the bottom of Ihyll. Very little remains of what could be assumed to have been a more bustling, populated labyrinth.
Fast forward to the current timeline: the students of Byrgenwerth explore the dungeons pretty thoroughly. They initially find phantasms, familiars of the Great Ones, and use them to gain rudimentary knowledge of the Eldritch Truth. They establish the notion of "Forbidden" blood through contact with Queen Yharnam, presumably locking her up in the process. They likely shook hands or otherwise exchanged greetings with Ebrietas at some point judging by the presence of the Augur of Ebrietas in the Byrgenwerth Lecture Hall.
Fast forward to the inception of the Healing Church, they do some unspecified exploration of their own in the labyrinth, both in Isz(as evidenced by the Loch shield) and Ihyll(as evidenced by NPC Tomb Prospectors), though it wouldn't appear there's any evidence of the Church in the Loran labyrinths. Perhaps this is why they naively continued using the Blood? They didn't have Loran to look to as a precedent, and so didn't properly fear the consequences of driving a whole civilization to beasthood?
Sources
Chalices
Pthumeru Chalice
A chalice that breaks a labyrinth seal.
But only a Root Chalice changes the shape of the
old labyrinth when used in a ritual.
The old labyrinth was carved out by the Pthumerians,
superhuman beings that are said to have unlocked
the wisdom of the eldritch Truth.
Pthumeru Root Chalice
Root Chalice that breaks multiple labyrinth seals.
Root Chalices, used in rituals to break old labyrinth seals,
are said to change the labyrinth's form each time.
The old labyrinth was carved out by the Pthumerians,
superhuman beings that are said to have unlocked
the wisdom of the eldritch Truth.
Sinister Lower Pthumeru Root Chalice
Root chalice that breaks multiple labyrinth seals.
When used in a ritual, this sinister chalice summons the Sinister Resonant Bell.
The bell-ringing woman appears to be a mad Pthumerian.
The old labyrinth was carved out by the Pthumerians,
superhuman beings that are said to have unlocked
the wisdom of the eldritch Truth.
Defiled Chalice
Chalice that breaks a labyrinth seal.
Only, this defiled chalice is cursed.
Curses are caused by inciting the anger of the Great Ones,
and used to hex others.
Special materials are required to complete the ritual.
To try one's hand at curses, first seek the Bastard of Loran.
Cursed and Defiled Root Chalice
Root chalice that breaks multiple labyrinth seals.
This cursed chalice makes a special ritual possible,
but must not be used lightly, as the resulting transformation can be utterly drastic.
The old labyrinth was carved out by the Pthumerians,
superhuman beings that are said to have unlocked
the wisdom of the eldritch Truth.
Great Pthumeru Ihyll Chalice
A chalice that breaks a labyrinth seal.
Great chalices unlock deeper reaches of the labyrinth.
Pthumeru Ihyll was the title of both the Phumerian monarch and its capital.
This reveals that while early Pthumerians were mere humble guardians
of the slumbering Great Ones,
their descendants felt entitled to name themselves a leader.
Pthumeru Ihyll Root Chalice
Root Chalice that breaks multiple labyrinth seals.
Root Chalices, used in rituals to break old labyrinth seals,
are said to change the labyrinth's form each time.
The Pthumerian monarch was traditionally a woman
who assumed a name with classical roots.
Hintertomb Chalice
A chalice that breaks a labyrinth seals.
Hintertombs are the peripheral catacombs of the old underground labyrinth.
To this day, the Watchers continue to expand the hintertombs,
unceremonious catacombs filled with graves and death.
Hintertomb Root Chalice
Root Chalice that breaks multiple labyrinth seals.
Root Chalices, used in rituals to break old labyrinth seals,
are said to change the labyrinth's form each time.
The hintertombs are unceremonious homes to graves and death,
cesspools of noxious snakes and insects.
Ailing Loran Chalice
chalice that breaks a labyrinth seal.
Loran is a tragic land that was devoured by the sands.
The tragedy that struck this ailing land of Loran is said to have its root in the scourge of the beast.
Some have made the dreaded extrapolation that Yharnam will be next.
Ailing Loran Root Chalice
Root chalice that breaks multiple labyrinth seals.
Root chalices, used in rituals to break old labyrinth seals,
are said to change the labyrinth's form each time.
There are trace remains of medical procedures in parts of ailing Loran.
Whether these were attempts to control the scourge of the beast,
or the cause of the outbreak, is unknown.
Great Isz Chalice
A chalice that breaks a labyrinth seal.
Great chalices unlock deeper reaches of the labyrinth.
The Great Isz Chalice became the cornerstone of the Choir,
the elite delegation of the Healing Church.
It was also the first Great Chalice brought back to the surface
since the time of Byrgenwerth,
and allowed the Choir to have audience with Ebrietas.
Isz Root Chalice
Root Chalice that breaks multiple labyrinth seals.
Root Chalices, used in rituals to break old labyrinth seals,
are said to change the labyrinth's form each time.
According to the Choir, the land of Isz lies in contact with the cosmos,
which allowed the Great Ones to function on transcendental planes of thought.
Short Ritual Root Chalice
One of the root chalices that breaks various seals in the Old Labyrinth.
The glass chalice, when full, is used in a short ritual to quickly create and join
a Chalice Dungeon (not for use in a ritual using standard materials).
Short rituals are conducted at the makeshift altar.
Materials
Arcane Haze
Material used in a Holy Chalice rite.
The tiny smatterings of haze that are found in certain ritual materials.
Sometimes required for special rites.
The additional rite Sinister Bell makes the bell-ringing woman appear,
and when she rings a sinister bell, hunters from other worlds will be beckoned as adversaries.
Bastard of Loran
Special material used in a Holy Chalice ritual.
Remains of Loran infant infected by the scourge.
A harbinger of curses and symbol of defilement.
The additional rite Curse defiles dungeons in which hunters' HP is greatly eroded,
but what better place to seek cursed blood gems but in the midst of defilement?
Bloodshot Eyeball
Material used in a Holy Chalice rite.
An exquisite eyeball removed quickly after death, or perhaps even before.
Used to unlock the seal of the old labyrinth hintertombs.
Blooming Coldblood Flower
Material used in a Holy Chalice ritual.
Pale vegetation that commonly grows on coldblood in a place long abandoned,
that has bloomed into a bright red stygian flower.
Coldblood Flower Bulb
Material used in a Holy Chalice ritual.
Pale vegetation that commonly grows on coldblood in a place long abandoned,
said to mature slowly in close proximity to death, and eventually bloom.
Coldblood Flowerbud
Material used in a Holy Chalice ritual.
Pale vegetation that commonly grows on coldblood in a place long ago abandoned.
Inflicted Organ
Special material used in a Holy Chalice ritual.
Special body parts collected by the Healing Church.
What was this gruesome bait used to lure?
The addition rite Rotted Offering beckons ungodly forces,
normally terrifying things, but also worthy prey for a special hunt.
Living String
Special material used in a Holy Chalice ritual.living_string.png
The immense brain that Mensis retrieved from the nightmare
was indeed lined with eyes on the inside, but they were of an evil sort,
and the brain itself was terrible rotten.
But even still, it was a legitimate Great One, and left a relic.
A living relic, at that, which is a precious thing indeed.
Pearl Slug
Material used in a Holy Chalice rite.
Of the all the strange lifeforms that reside in the nooks and crannies of the old labyrinth,
the slugs are clear signs of the left-behind Great Ones.
Red Jelly
Material used in a Holy Chalice ritual.
Stillborn infants born of a creature of unknown origin,
of the type found in some corners of the old labyrinth.
Ritual Blood
Material used in a Holy Chalice rite.
One of the basic ingredients used to satiate a Holy Chalice is this incoagulable blood.
When all is melted in blood, all is reborn.
Sage's Hair/Wrist
Special material used in a Holy Chalice ritual.
A body part of a saint, sacrificed by the Healing Church in their search for Truth.
The additional rite Fetid Offering invigorates the Watchers,
making them more treacherous than normal,
but also making them worthy prey for a special hunt.
Tomb Mold
Material used in a Holy Chalice ritual.
Mould that grows from rotten flesh and blood inside the old labyrinth.
Matures to bear giant spores.
Yellow Backbone
Special material used in a Holy Chalice rite.
Special body parts collected by the Healing Church.
What was this gruesome bait used to lure?
The additional rite Rotted Offering beckons ungodly forces,
normally terrifying things, but also worthy prey for a special hunt.
Other
Ring of Betrothal
The inhuman beings known as the Great Ones imbued this
ring of betrothal with some special meaning.
In the age of the Great Ones, wedlock was a blood contract,
only permitted to those slated to bear a special child.
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