So, in Bloodborne, there appear to be many different iterations of how the Great Ones are "created".
For example, Rom is only a Human that "ascended" (apart from the Hunter in the "Childhood's Beginning" ending), though it is argued that it was not a full transformation, classing her only as a "transformed being"
Rom, the Vacuous Spider controls the barrier that prevents normal humans from seeing the true horrors around them, such as the Lesser Amygdalae crawling around Cathedral Ward. Rom was once a scholar of Byrgenwerth who at some point went through metamorphosis into a Kin courtesy of Kos, according to dialogue given by Micolash.
This also has something to do with Ebreitas, who worked with the Healing Church, teaching them ways of Blood Ministration, etc., and when found in the Altar of Despair, appears to be "mourning over Rom's body."
When the player finds her she seems to be mourning (or simply sleeping) at the Altar of Despair, which appears to house the corpse of a dead being that resembles Rom.
Mergo, on the other hand is the offspring the child of the Pthumerian Queen and Formless Oedon
Mergo is an infant Great One, suspected to be the child of Yharnam, Pthumerian Queen and Oedon, a Great One.
The child appears to have been stillborn in some fashion, as evidenced by the item description of a certain Third Umbilical Cord, "Every Great One loses its child, and then yearns for a surrogate. This Cord granted Mensis audience with Mergo, but resulted in the stillbirth of their brains."
Oedon himself seems to have attempted this multiple times, as well.
Oedon might have used Arianna's position as a prostitute and her origin as a distant relative to Queen Annalise to use the blood she was exchanging with people to impregnate her. The latter seems to be more correct since the Third Umbilical Cord collected from her child states: "Every Great One loses its child, and then yearns for a surrogate, and Oedon, the formless Great One, is no different. To think, it was corrupted blood that began this eldritch liaison."
Then we have Kos, and the Orphan of Kos. This seems to be the only Great One that directly births an Offspring, without using a human surrogate, to somehow create a new Great One. There is nothing that I have found to state anything about how Kos became "pregnant", only that The Orphan of Kos was birthed from her dying body, simply because it's a cutscene to show it happening.
Then finally, there is the Moon Presence. This is probably just as vague; the only clues are that the consumption of the Umbilical Cords give the Hunter the ability to resist the will of the Moon Presence. Then, upon defeating it, you simply become an Infant Great One, apparently fully ascending to the proper status of "Great One", unlike Rom.
The Hunter succeeds in defeating the Moon Presence, and ascends into an infant Great One, that is found and cared for by The Doll.
So, is there a "process" that allows for the creation of a Great One? And if so, what are the requirements?
