The following content was originally created in this thread on Reddit in /r/bloodborne. All credit for the content of this post goes to /u/tomb-inspector, with only minor changes being made to reflect the difference in format.
In this post, /u/tomb-inspector examines the history of the Great Ones and their path to transcendence.
|Image taken from Fextralife Bloodborne Wiki|
The Ascension of the Great Ones to Godhood
“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” - Isz Root Chalice
Transcendental planes of thought here refer to planes higher than the physical world, in other words, the world of dream/nightmare. It says that the contact with cosmos allowed the Great Ones to function ("exist/operate") in the nightmare. It implies therefore that the Great Ones do not always exist in higher planes, but ascended to them at some point in the past. Before that, the Great Ones exist only in the physical world.
“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” – Ring of Betrothal
I would venture to assert that the age of the Great Ones referred to here is in fact describing this period. It means the age when the Great Ones ruled on earth. Who are their subjects? The Pthumerians. During this period, the Great Ones established the institution of marriage as a “blood contract” to produce a “special child”. Outside of wedlock, the Pthumerians may have offspring freely, but those chosen to bear a special child, are bound by wedlock (blood contract).
My take here is that, each Great One is unique (its own species), and the only way for them to have an offspring is through the Pthumerians via a blood mechanism. Whether infusing a Pthumerian child with its blood, or the mother before conception, or the couple, or all of them, or even just directly impregnating a female Pthumerian is unclear. The mechanism may even differ from one Great One to another. The resulting offspring survived, and each Great One had its own child.
With the discovery in the land of Isz came the possibility of the Great Ones’ ascension to higher planes. They decided to go for it, possibly because in the physical world, they were not truly immortal. During the process, they put their physical body and those of their children to slumber and ascend their consciousness to the nightmare. The children however, since they are not pure Great Ones, did not survive the ascension. Their bodies lied in slumber, yet their consciousness did not reach the dream world.
“Every Great One loses its child, and then yearns for a surrogate” – One Third of Umbilical Cord
Not all Great Ones ascended however.
“Of all the strange lifeforms that reside in the nooks and crannies of the old labyrinth, the slugs, are clear sign of the left-behind Great Ones.” – Pearl Slug
“Use phantasms, the invertebrates known to be the augurs of the Great Ones, to partially summon abandoned Ebrietas” – Augur of Ebrietas
One such Great Ones is Ebrietas, the abandoned/left-behind Great One discovered by the Choir in the labyrinth of Isz. Why was Ebrietas left behind or abandoned in this basest of existence? I would suggest that, it was her own decision not to ascend, for the love that she had for her daughter, Rom. She couldn’t bear to be separated from her, even if the reward was godhood, and so she remained.
“...might the cosmos be very near us, only just above our heads?” – Cosmic Eye Watcher Badge
“Together with the left behind Great One, they look to the skies, in search of astral signs, that may lead them to the rediscovery of true greatness.” – Choir Garb
In time, the connection to the cosmos was lost, and the Choir looks to the skies (who they assume to be cosmos), together with Ebrietas, in the hope of rediscovering the link to true greatness/godhood/cosmos.
What happened to the Pthumerians after the ascension event?
“Pthumeru Ihyll was the title of both the Pthumerian 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.” - Great Pthumeru Ihyll Chalice
At first they humbly kept watch over the physical remains of the slumbering Great Ones (the tombs of the old Lords) with no other ambitions. But over time, with no more Great Ones to rule over them (most of them ascended), they elected a ruler, a queen (the new Lord), built cities and established the Ihyll monarchy. Thus began the Pthumerian civilization.
EDIT [ADDITIONAL MATERIAL IN RESPONSE TO COMMENTS]:
Ok, I have somewhat organized some thoughts. Let me try to expand a little bit more on this part, and answer some of the objections raised to my theory regarding Rom-Ebrietas here.
Was Rom formerly a human?
I don't think so, for the following reasons:
a. Rom was found in the Chalice dungeons (old labyrinth of the Pthumerians). Most of the stuff found in the dungeon (except those dropped by dead/mad hunters/explorers & beasts who managed to venture deep into the tomb), traced their origin back to the ancient Pthumerians. I conclude therefore that Rom existed from the time of the Pthumerian (even going back to the age of the Great Ones). I would even suggest that the first contact that the Byrgenwerth scholars had with a significant eldritch entity is in fact, Rom, for she can be found quite early in the chalice dungeons. The scholars then brought Rom back to the surface, to Byrgenwerth, to be studied. Just like how the Choir later discovered Ebrietas, and brought her to the surface. The discovery of Rom led to the truth that: first, the Great Ones exist, and two, evolution from lower beings to Great Ones is possible, and Rom is the living proof of that. This brings me to the second question below (after b.).
b. The name Rom is not mentioned anywhere in association with Willem, Laurence, Gehrman, or in association with the students/scholars of Byrgenwerth. And no other name/nameless person reference can be linked to Rom. (I might be wrong on this)
Was Rom a Great One?
Yes, in the derivative sense. She was indeed a being that had evolved somewhat to a level of a Great One. But not from human, instead, from a Pthumerian (possibly a Pthumerian child, seeing how she acted and moved during battle). The Pthumerians are super-humans, and maybe the precursor to humanity. Like I mentioned in the OP, she was a product of the blood contract between a Great One and the Pthumerians to produce an offspring for the Great One. Given this, I will interpret Micolash speech as saying: "As you once did for the vacuous Rom (the Pthumerian), grant us (also, humans,) ..."
The Byrgenwerth scholars disagreed on how to achieve what Rom had achieved. Willem, probably from seeing how Rom was covered with eyes, concluded that eyes are the key. Laurence disagreed, to him blood is the key.
Now if Rom is a child of a Great One, kin of Cosmos, brethren to the Great Ones, which Great One is she the child of? I concluded that it's Ebrietas because:
a. After killing Rom, we find Ebrietas in front of an altar with something that looks like Rom, and she seems to be grieving to the point that we have to be the one initiating the battle.
b. Rom, although she is referred to as a spider, looks like a caterpillar to me. And I read somewhere before that the name Ebrietas is a species of butterfly and indeed the design of Ebrietas somewhat resembles a butterfly. Well, I make the interpolation that since a caterpillar evolve to a butterfly, Rom would eventually evolve to be like Ebrietas.
Well, this last point is not convincing at all, even to myself, lol. And someone pointed out that Ebrietas might also be an evolved Pthumerian, which I think is possible. Anyway, these are my thought process.