What is The Eldritch Truth?
Spoilers Ahead...
Numerous times in Bloodborne, there is mention of different people encountering 'the eldritch truth,' including the Byrgenwerth scholars, the Pthrmarians, and even an anonymously mentioned doctor and his sister. Some are glad to encounter it, and nearly all who encounter it have gone mad to some degree. But what could it be? It seems to be connected to the Kin, but I don't know for certain. Is it the simple knowledge of the Great Ones? Was it the finding/killing of Mother Kos at the Fishing Hamlet years ago, and had those fishermen been blessed by it? Or is it something more (or less)?
What is the eldritch truth?
I got this about halfway down the Bloodborne Wikia:
The idea of the Eldritch truth is spread throughout the game as people from Master Willem, to even the choir (a group of people), have tried to achieve an evolution toward a higher state of being by learning the Eldritch truth. The truth it is not expressed by simply a sentence or a phrase, rather it is a state of being in existence. The great ones understood this either through naturally knowing the eldritch truth, having been humans or beings which gained enough insight to evolve, or an otherwise unknown method to understand the true state of reality. Later as you, the player, by killing Rom made the true reality of the world shown (for an arbitrary reason his* [sic] death either as part of a ritual or by being the thing holding back people from seeing reality. (This needs to be confirmed or greatly defined)) or by gaining 40 insight you saw the reality of the world as it truly was, and achieved what those before you sought to do but failed. They failed by using other methods like lining your brain with eyes, using the Mensis cage, or accepting a great ones power many of which failed to achieve true enlightenment as a side effect resulted in madness, deformation/transformation into a monstrosity, or death for the user with their corresponding method.
However you, as the player, achieved this by gathering insight throughout the world and absorbing it into your itself. Following this, utilizing the action of using the item in game you crush the skull in your hand releasing this partial knowledge into the world which channels into you for some reason either because of proximity or another variable. Other ways such as fighting nightmarish enemies which provoke your mind into perceiving them, which forces you to gain insight so you can fight them it is also a safe method of gaining insight. This way you don't get side effects (as far as I know if you find any then add them). This makes you able to perceive the great ones fully, and able to see the world as it truly is plus interacting with the real reality; such as being able to enter and face enemies in the Yahar'gul Unseen Village under the blood moon which was previously not fully available. The difference in the area as far as how deep you can go into it between the variety of enemies is shown in a comparison of the area and when you are captured by the Snatchers and taken to Hypogean Gaol as well as after you visit the area and have killed Rom. The player thus gain's true enlightenment or simply understand the eldritch truth. As well on a final note, you are probably wondering how blood ministration fits into this explanation well in the Bloodborne universe blood is used as a cure/healing or a way of making a individual stronger. Therefore, in a sense it is moving towards this same goal although from a different direction. It ultimately never leads to the Eldritch Truth as the humans wanted (ideally they got to be elevated, understand more with a clear mind, and be in control of themselves), just a monstrous transformation but considering many of these enemies end up in the true reality you could say they reached the physical state of enlightenment but their minds are completely gone in most cases and they just attack anything they can. So they got what the wanted they just can't enjoy it.
*Canonically, Rom the Vacuous Spider is female.