Certain info on Bloodborne I figured out from watching the game on YouTube and reading the Wikis:
Spoilers
Bloodborne has interesting Lore concerning the Great Ones:
The Beast Scourge
The whole Hunter business is tied to a conflict between Kos and the Moon Presence. Over what you ask? The Moon.
As anyone whose seen the Old Hunters DLC knows the Hunter’s Nightmare created by Kos in retaliation for her death has a unique Moon different from the ordinary Moon or even the Blood Moon of the Moon Presence.
Since the Moon Presence has claim to the Moon the tampering by Kos is pretty much desecration and demands death. How does the Moon Presence deal with this? The First Hunter Gehrman and the Vileblood Hunters he established using the Blood Dregs gathered from Hunters.
Gehrman became the first Hunter by being exposed to the Old Blood of Ebrietas who is the Daughter of the Cosmos or should I say Daughter of the Kosmos.
The Moon Presence made it so that the Daughter of Kos’s gift of the Old Blood would immediately become tainted with Blood Dregs when injected into people turning them into Hunters.
The Blood Dregs were used by Gehrman to create the Cainhurst Vilebloods one of which Lady Maria joined Gehrman in killing Kos at the Moon Presence’s orders inducing a curse by Ebrietas’s brother the furious Orphan of Kos that resulted in the Beast Scourge.
As a result Byrgenwerth banned the use of the Old Blood yet Laurence left Byrgenwerth and established the Healing Blood Church believing that as long as the Hunters did not use Blood Dregs they would not be punished by Kos.
Unfortunately we will never know if that would have been true as Ludwig the Holy Blade discovered the Holy Moonlight Sword which as one can guess was tied to the Moon Presence.
The Moon Presence through the Sword induced Ludwig to make it so that Ludwig would turn the Townsfolk into Hunters increasing the amount of Blood Dregs linked to the Moon Presence for the Vilebloods to acquire and use. This heresy against Kos sealed the fate of any who used the Old Blood regardless of whether or not they used Blood Dregs.
Unless the user of the Old Blood had Insight they would become a Beast the second they act thoughtlessly like a Beast.
The Hunt:
The Moon Presence only lets the Player Hunter leave the Hunt after they prevent the School of Mensis from gaining an audience with Kos via taking away their Umbilical Cord. To even acquire the thing they had to eliminate a Mortal ascended by Kos into a Great One and actively preventing a Blood Moon from appearing.
The Moon Presence rewards the Hunter for acquiring the School of Mensis’s Umbilical Cord by letting them leave the Hunt or if they kill Gehrman in order to stay in the Hunter’s Dream continued service under it’s will.
Of course if the Hunter consumes 3 Umbilical Cords the Hunter will start becoming a Great One and when the Moon Presence discovers this it immediately comes to the conclusion that the Hunter seeks to usurp it like Kos attempted to usurp the Moon and seek to kill the Hunter personally.
The Hunter kills the Moon Presence and gains dominion over the Hunter’s Dream while also avenging Kos halting the spread of the Beast Scourge(if killing the Angry Orphan of Kos didn’t already do that).
The Kin of the Cosmos
The Kin of the Cosmos are Kin of Kosmos in some manner either through being ascended by Kos or descended from Kos.
Ebrietas is Kos’s Daughter while the Orphan of Kos is Kos’s Son and both Bleed Pale colored Blood as do other Kin. The Orphan of Kos of course is immune to Kin-harming weapons as are the Deep Ones whom Kos created.
The College of Byrgenwerth sought to grow Eyes following the atrocity by the Hunters and Vilebloods and thus first tested by getting Kos to turn Students into Gardens of Eyes followed by experiments to grant Eyes on the inside resulting first in the Headmaster becoming barely intelligible followed by the first success in Rom who became a Great One.
The Healing Church after being established tried to continue the plan by preforming experiments to evolve Humanity first creating the Living Failures whom lacking the fine control of Ebrietas created meteors instead of Bolts of Celestial Energies.
The Healing Church ultimately had more success when they began testing on children which they used to create Celestial Emissaries with the only one that didn’t turn violent becoming a Great One like Rom did.
The Healing Church’s experiments also resulted in people giving birth to larval members of Ebrietas’s species.
School of Mensis
The School of Mensis in Yahar’gul the Unseen Village acquired Mergo for the sake of his Umbilical Cord and had it guarded by Mergo’s Wet Nurse.
The Cord was used in an attempt to turn the School of Mensis into Great Ones. They turned most of the School into the Brain of Mensis who resembled a brain heavily empowered by Insight.
Micolash leader of the School of course was in the middle of trying to ascend like his Students and Rom but the Player Hunter killed him as well as the One Reborn(a failed attempt to create a Great Ones), the Brain of Mensis and Mergo’s Wet Nurse acquiring the Umbilical Cord.