Disclaimer: Contains spoilers for Bloodborne.
There’s no question that the Hidetaka Miyazaki game Bloodborne is heavily influenced by the works of H. P. Lovecraft, particularly the concept of the Great Ones and other incomprehensible beings. There are three main cycles in Lovecraft’s work, with some overlaps here and there: The Cthulhu Mythos, the Dream Cycles, and his miscellaneous works. For this theory we’ll just be focusing on the first two.
There are two factions of “Gods” (for lack of a better word) in Lovecraft’s mythos: The Great Old Ones, such as Cthulhu and Tsathoggua, and the Outer Gods, such as Nyarlathotep and Azathoth. (There are also their minions, but those irrelevant here.) The Great Old Ones are mostly Earth deities, worshiped by ancient man. They are rooted in nature and the workings of the Earth itself. Then, there are the Outer Gods, which are more concerned with the Dream Lands and space. They exist outside of our natural realms, living instead in the far reaches of the cosmos and other dimensions.
In Bloodborne, the main goal of the humans is to transform into higher beings, akin to the Great Ones, through consumption of the blood of the left behind Great One, Ebrietas and experiments with the Kos parasites. However, these humans attempting ascension seem awfully confused. They believe that the Kos parasites will grant them the insight needed to ascend, and begin working with water-based experiments. Kos seems analogous to the Great Old Ones of the Earth. However, there seem to be bigger fish to fry, like the Moon presence, which seems more like an Outer God. It’s not a one-to-one comparison, but I believe Bloodborne drew from this concept of there being two factions of Gods, and created a conflict between them. Someone* suggested that the Amygdala seemed to be the antagonists of the Moon Presence.
*Author’s note: My sincerest apologies, I can’t recall whose theory this is. If anyone reading this knows, please bring it to my attention in the comments below so I can properly attribute their work to them.
Furthermore to prove that the citizens of Yharnam are greatly unaware of the truth, they mistook the concept of Insight to mean “Eyes on the inside.” This makes no sense, honestly. It sounds like the result of a game of telephone, wherein the original concept was to gain insight into the nature of these greater beings, so as to reach their level of existence, but over the years, or decades, or possibly even eras, this word “insight” was mistranslated by those that followed the original discoverers of the eldritch truth, and thought that they were supposed to literally have eyes inside their brains. They therefore began experimenting on animals, and later humans, mutilating them to sprout multiple eyes.
My theory on what all this means for the story of Bloodborne: Humans attempting ascension are concerned with the “Great Old Ones” like Kos, but they fail in their goals because this is not the true way. The twist at the end is that, while the humans were concerned with the Great Old Ones, the Moon Presence, an Outer God, was pulling the strings the whole time, and it is the true means of ascension. The ending is a revelation that everything the humans thought was important was a red herring. It was never the way to becoming a higher being. Something that other humans could never have fathomed was there was the true means they sought. It enslaved Gehrman so that it could find a Hunter to do its bidding, and destroy the Great Old Ones for reasons that are probably unfathomable to us mortals. Who knows how long this has been going on. Perhaps every Hunter has been given a different task, each one a step towards the Moon Presence’s ultimate goal. Eileen and Djura seem to have been successful in whatever task they were given, and were returned to the waking world by Gehrman’s execution.
Notice how it’s only once you meet the Moon Presence that you ascend to being a higher being. That was the only way the whole time. The blood of the Great Old Ones, Gods of the Earth, were only ever going to turn people into beasts, because the beasts are creatures of the Earth and Nature. The only way to become a higher being, is to commune with the cosmic entities.