Item descriptions, lad. You must read them all.
But here's the gist of it:
Mankind has wanted a way to transcend humanity. Different people have different opinions on how to reach this point however.
The College of Byrgenwerth seeks transcendence through insight. They think that if they can obtain the knowledge of the Great Ones then they can become them or beings like them. In order to see beyond the ken of mortals and to that of the Great Ones they acquire eyes - literally lining the inside of their heads with them. Under the guidance of Provost Willem, they, "Fear the Old Blood," and never delve too deep - despite Willem using a third umbilical cord to commune with the Great Ones. Perhaps as a result though, Rom comes to reside at Byrgenwerth to conceal Willem from the rest of the school as if to keep forbidden knowledge hidden. Among this knowledge is that of Queen Yharnam of Pthumeru, her stillborn Great One infant Mergo, and the existence of the Blood Moon.
The Healing Church, founded by Laurence after he split from Willem and Byrgenwerth, uses blood as method of communing with the Great Ones. This stems from the fact that the Great Ones desperately want to have children but cannot give birth to live progeny, seeking surrogate mothers such was the case with Queen Yharnam of the long dead Pthumerian civilization. Through communion, members of the Church are blessed with transfigured blood that holds curative qualities. However this turns them into beasts along with those who have been administered the blood.
The School of Mensis utilizes dreams to directly contact a Great One. In particular, they devote themselves to the Moon, "Mensis," being Latin for, "Month," lunar cycles historically having been used as the basis for months. The School lures a Great One to them for communion by using a third umbilical cord along with their cages so that they enter the Great One's domain within the Nightmare Frontier. The Great One they contact is Mergo, an infant born from Queen Yharnam of the Pthumerian civilization. Only Mergo was never alive, stillborn like all other attempts by the Great Ones to have surrogate births using human mothers. This contact destroyed the school, however it created a Great One from their brains, the Brain of Mensis, and it is this brain that gives birth to the Nightmare of Mensis, overseen to a degree by Micolash. One other thing of note is that the School of Mensis and the Healing Church are enemies, evidenced by a dead Choir member locked up by Mensis and a Choir spy gathering intelligence within the Nightmare of Mensis.
The Hunters don't seek communion, but the Hunter's Dream which they find refuge was created from communion with the Moon Presence also know as Paleblood. This was achieved though Gehrman's use of a third umbilical cord. Hunters within the Dream are locked in a cycle of killing beasts during an unending night. The scourge of beasts they seek to end points back to the Blood Moon which is connected to Mergo and the Moon Presence, and by destroying Mergo's Wet Nurse, a hunter may be freed from the Dream by Gehrman, joining other hunters such as Eileen and Djura who also no longer dream. Of course this also dooms hunters to eventually become beasts when the Pale Moon rises again. The order of events remains somewhat unclear though, as does the causes of certain events. For example, the Blood Moon may have been beckoned by the School of Mensis when they enacted their ritual to commune with Mergo, and this may have caused the scourge of beasts to descend upon Yharnam. What seems fairly certain is that the Moon Presence has claimed Gehrman like an adopted child, and by killing him and freeing him from the Dream, the Moon Presence embraces the hunter who inherits Gehrman's role within the Dream.
The true ending involves the hunter using three third umbilical cords to gain such a level of insight that the Moon Presence can be resisted and killed. This is the transcendence that everyone was looking for. They didn't go far enough like Byrgenwerth and the Healing Church or jumped the gun and tried to transcend too soon like the School of Mensis whose minds were not prepared. The importance of the cords cannot be overstated in this regard. By using three, one's thoughts are elevated to the level of a Great One. Then by seeking Paleblood, the wishes of both humans and Great Ones are fulfilled: One transcends humanity without becoming a beast or being overcome by insight, and an infant Great One is finally born. In a way it can be seen as putting all the pieces together, utilizing blood, insight, and dream.