My understanding and speculation is filling out loosely like this, let me know how you think this highly metaphysical shit fits together or if you think it specifically conflicts known facts/timeline: -The old ones want to reproduce. The specific mechanics of how this ideally works are still being figured out, but it's an arcane process that involves manipulation of human minds and bodies through old ones' Blood and Truth, is not usually (or ever) successful, and usually leads to them compensating by claiming a surrogate directly from a human host. -In Pthumeria this process was held as a sacrament using the pale blood of the old ones and great pains were taken, through rites and rituals and religion, to keep these sacraments under control and worshipfully facilitate the great ones' wishes. -In Brygenwerth, the process was corrupted when humans tried to use the sacraments of the Blood and the Truth to empower themselves to their own ends. As a result of this (from the old ones' perspective) miscarriage, Micolash threatens human reality, with the end result that as damage control, Rom (as an inverse-maternal symbol) guards the seal on the reproduction of the old ones through the full moon and the lake (both as womb symbols). -During this time the Hunters rose to combat the beastly collateral damage of the nightmare miscarriage, possibly with the hidden agenda of finding and destroying Rom and unsealing the old ones' true reproduction. I believe that Hunters had (or developed) a separate set of sacraments for the blood, but are still highly disciplined compared to blood use in Yharnam. Where Pthumerians were showing themselves as willing vessels to the old ones, hunters are weaponized in their service, and their use of the blood is to give them power, speed, enhanced senses, and empathy with their prey. -In order to use the blood without falling victim to it, the Hunter's Dream was initialized, through the cooperation of the old ones and with Gherman as its kernel or foothold or first subject. It is an oasis for Hunters within the chaos of the Blood's influence, and Gherman's spirit and discipline are holding it open and providing a template. It would make sense if his being sealed in the Hunters Dream correlated with the seal on the reproductive process by Rom, but I'm not sure the timing on that. -In Yharnam, The Blood is used for its power without sacrament or discipline, and the corruption of an unprepared and undisciplined human host is almost guaranteed. Beasts run wild, Hunters fight them, Old Yharnam burns, the rise of the Healing Church.... all mostly collateral damage from the Blood leaving Byrgenwerth and the Hunters continuing their mission of cleaning up nightmare fallout and freeing the old ones' womb. -The old ones are still desperate to reproduce themselves and still need humanity to do it. Their metaphorical womb has been sealed, but their blood still speaks to humanity and can bend humans' will, so there are elements of the Healing Church and Hunters who still tend towards their wishes. -As a result of all the complications to the original Pthumerian process before and during the game, the player character becomes the first successful "true" offspring of an old one in the third ending, birthed from the womb of the Hunter's Dream and repairing the corruption of the nightmare miscarriage. The two other endings represent either being trapped forever in or freed from the Hunter's Dream without being "born." Given that you've killed Rom and unsealed the nightmare, the two endings that don't see you fulfilling the reproductive cycle probably result in the Nightmare destroying human reality after the events of the game.

Even more rampant total speculation: "The Blood" and "The Truth" may be analogues to the old ones' reproductive cells, and bringing them together through the right medium and in the right incubator are the key?