So Demon's Souls is the first game. As we come to find out, The Old One isn't something brand new. Instead, it's something that has happened before, and as the Monumental tells you, they challenged it before and defeated it. But if you look around, you see that he is certainly not the first Monumental. There are a LOT of bodies there.
So we can take from this that The Old One has probably been called before and every time it has been beaten back. This is where the concept of magic came from, as well as pyromancy and miracles. Because as we see, humans learn all of their magic powers from the demons that come with The Old One.
But most importantly, this is where the power of the Soul first came into being.
Then, one time, The Old One won. Don't know when, don't know how. But one day, humanity failed to put The Old One back to sleep and it's fog covered the entire world. As we see in Demon's Souls, humans that are engulfed in the fog eventually go mad. So what happens when the entire world in engulfed? When there is no safe place, and humans are slowly reduced to worse than husks. Almost like...Hollows.
The only race that was unaffected by the fog was the Dragons. For some reason--don't ask, just fill it in--their scales protect them from it. As such, they slowly begin to rule over the world. But the fog doesn't leave them completely unchanged. Their scales shift from absorbing the fog's power, granting them immortality.
This goes on for time beyond memory, to the point where humanity is nothing but stumbling Hollows hiding deep underground in forgotten caves, wandering in the dark. They spend so many lifetimes in the dark that they forget light, but The Old One's fog still creeps down there. It is, after all, in the air. As such, the dark is twisted by the the fog until it becomes The Dark and since the Hollows constantly exist in this muck, they become one with it. It becomes a part of their being, forming what would later become known as Humanity.
Now, what happened to The Old One? Not sure. Maybe the Dragons tired of it and killed it. Maybe it died. Who can say? All I know is that it did die, and that random skull down in Ash Lake is all that's left of it.
Now this leaves us with a certain character from Demon's Souls who suddenly doesn't have a purpose anymore: The Maiden in Black. With the death of The Old One, she no longer has anything to do. So I think she wandered the land alone for eons. She learned the Dragons' language and probably spoke with them in order to pass the time. And I think that maybe she even had a child with one of them--I know, I know. Hold on, I'll get to why in a bit.
But the important thing is that I think she did have a child, but the Ancient Dragons shunned her child because it wasn't a pure dragon, and thus shunned The Maiden in Black too.
And this is where things get interesting. I think it's the Maiden in Black who showed the Hollows down below Fire. Remembering the power of Soul Arts from eons before, she went down and created The First Fire as a means to get even with the dragons for their betrayal of her and her child.
You know what comes next: Dark Souls. Gwyn, the Witch, Nito, and Seathe rise up and overthrow the Dragons and build their own world. But they forget who helped them. The forget that it was the Maiden in Black that showed them Fire. Instead, they come to fear her, this ancient being who showed them a power that could crush the power of the Dragons. And if she's that strong, what could she do to them? To these gods? And so they begin to speak of her in hushed tones, alongside her child. They come to know this being dressed in black by a certain name:
Velka.
And who is Velka's child? Priscilla. Why do I think this? Because it says that Priscilla has the ability that even the gods feared: the Life Drain. Now, as we are lead to believe, Life Drain is the blood loss ability, or bleed effect, but I've never really understood that. Why would the gods fear something that just a normal weapon has too?
Now, what does The Maiden's soul DO in Demon's Souls? That's right. It sucks the life out of its target, tearing their Soul. And who's to say that Velka's child wouldn't have this same terrifying power?
So, Dark Souls happens, and Gwyn and Witch mess with The Maiden's/Velka's Fire, and Velka gets fed up and turns to humans, setting them loose upon the gods as well now.
Time goes on and Dark Souls II happens, and then Dark Souls III happens, but something important happens in Dark Souls III that will change the world, just as the discovery of the power of the Soul from the demons did in Demon's Souls.
And that something is Aldrich.
As you recall, Aldrich did something no one else had ever done: he ate a god--possibly even more than one. And in doing so, he had a vision of the deep sea. Sounds like something from Bloodborne, right? But wait, there's more.
Over the eons, people forget Aldrich, but they remember the eating part. Eventually, this belief gets twisted into consuming or drinking Blood. People are no longer consuming the Soul itself, but rather the Blood that holds the power of Soul in it.
And so, Bloodborne, and its thing with Blood, is born.
Now, who are these Great Ones, like The Moon Presence and others? I mean, it says they came from space. Well, as you see in one of Bloodborne's endings, a human CAN become a Great One. So who's to say that these Great Ones that supposedly came from space are actually powerful beings that ascended just like you do? Only they did it sometime looooooong ago, after Dark Souls III but long before Bloodborne, and their power became so great that humanity could only begin to understand them by saying they were from a different world.
