As recent blog posts have reinforced, I adore Bloodborne, FromSoft's Gothic/Cosmic Horror reimagining of their Demon's Souls/Dark Souls gameplay formula, which released in 2015 and is considered, I think to be one of their masterpieces (I'm personally of the opinion that, when combining gameplay and story, Elden Ring is their current crowning achievement, but it is obviously standing on the shoulders of giants, so to speak).
The Ravenloft setting for D&D is very different than others. While Greyhawk, Forgotten Realms, and Dragonlance are all worlds with fantastical histories, they're still basically "real" worlds, which are bound by the logic and order of a realistic world that happens to also have magic.
The Domains of Dread, which is where Ravenloft takes place, are not.
While Ravenloft pre-dates the Shadowfell as a concept in D&D, it's never been in the Prime Material Plane (which is basically "reality a mortal is typically familiar with,") and was sort of afloat in the Ethereal Plane. Even under current lore, the Domains of Dread are a part of the Shadowfell that is separate from the main version of that plane (though as the most popular part of the Shadowfell, you don't often hear much about the rest of the Shadowfell in 5E materials). This is not our baseline reality, but it's also not the realm of the gods like the Outer Planes or the elements like the, well, elemental planes.
The Shadowfell is sort of a dark mirror of the Feywild. Both are realms that have their potential for horror - a spooky, gnarled wood that is home to curse-crafting hags could very well be part of the Feywild - but the way I see it is that the Feywild is full of supercharged emotion, while the Shadowfell is cold and drained, where motivation is sapped to become more compulsive and numb.
The Domains of Dread are specifically created by the Dark Powers - mysterious entities that could be some sinister cabal or might be unknowable eldritch beings from beyond - who pluck particularly evil people from the multiverse and create ironic prisons that are a reflection of their primary prisoners. Each domain has a Darklord, who is both the ruler of the domain and its primary prisoner, and is endlessly tormented as they seek to get their heart's desire but find those attempts thwarted. But it's not only the wicked who are tormented in these realms - the living people who inhabit the Domains, either born there or brought there through the Mists, which can reach into other planes and snatch you up, are forced to contend with these nightmare realms.
In its initial form in the early '90s, Ravenloft actually had something of a "world"-like landmass - a continent called The Core that placed all the domains of dread in geographical relation to one another. 5E's Van Richten's Guide to Ravenloft gets rid of this idea, instead putting the individual domains adrift in the Mists as separate little nightmare realms. This, incidentally, is something I really prefer, and whenever I hear people complaining that the book somehow "ruined" Ravenloft, I cannot take them seriously, because I hate the idea of The Core with a passion. /End Rant.
Now, each domain is stuck in an endless cycle. The domains won't let the Darklords die, perpetuating their torment. Sometimes, a group of adventurers will have to kill a Darklord in order to escape the domain, because each Darklord has a way to "close" the Mists surrounding their domain (something I interpret as not even necessarily being a conscious choice on their part). But they will always reappear after time, and continue to perpetuate the horror within their realm.
So let's look at Yharnam.
The notion of a cycle in Yharnam is certainly right there in the story: Hunters are sent to slay Beasts in the city, but those hunters then become infected with the Curse of the Beast, becoming the next threat that needs to be hunted.
This being FromSoft, you never know all the details completely, but my basic understanding of the "modern" history of the city of Yharnam is as follows:
Scholars at the old College of Byrgenwerth discovered an ancient labyrinth underground, which held the remnants of a pre-human (or proto-human) civilization called the Pthumerians. Within the "Tomb," they found a source of blood that could be used to heal any injury or illness. What that source is remains vague - I think you could possibly imagine that it involves the unborn child of Queen Yharnam, the Pthumerians' leader, who is probably called Mergo, and is likely the child of a Great One - an eldritch being from beyond familiar reality.
Discovery of that tomb led the scholars to try to contact and learn about the Great Ones, attempting to bring about an evolution of humanity - either culturally or even physiologically. Master Willem, the head of Byrgenwerth, wanted to learn more about the cosmos and the Great Ones, but to do so in a cautious and careful way (which still involved, like, adding eyes to the inside of their skulls).
But it was Laurence, one of Willem's students, who saw the potential in the Great Ones for a different path: to use the healing blood to found a religion, worshipping the Great Ones as gods and bringing fabulous prosperity to the city through the business of "blood ministration," healing sicknesses via the blood.
The biggest question, I think, if you wanted to adapt Bloodborne as a Domain of Dread, is who the Darklord would be. FromSoft sometimes makes some undeniably evil characters, but usually its primary antagonists are misguided more than evil (at least depending on your interpretation). Ravenloft's Darklords are all explicitly irredeemably evil (though as with anything D&D, DMs are free to change those rules as the story demands). I think that, given his central role, an argument could be made for Gehrman, but I think you could also argue that if he is a Darklord, it's of a different, related Domain - the Hunter's Nightmare. The evil he committed was probably the killing of Kos (if she didn't just wash up on the beach dead) or the massacre of the Fishing Hamlet. Kos and the Fishing Hamlet's role in the larger story is also pretty vague - I think the current read on it that I'd consider is that Gehrman's attack on the Fishing Hamlet probably led to Byrgenwerth's discovery of the Pthumerian Tomb - maybe finding out that Kos was a thing spurred forth the research into the Great Ones. (Was Byrgenwerth built on the ruins of the Fishing Hamlet?) Clearly, if Gehrman was the first Hunter, the Hunter's Nightmare was the fish-peoples' curse on them for his deeds. But I think it's likely this is a side-story.
Laurence, thus, to me, feels like the more obvious Darklord. The entire city feels like it's suffering because of his actions, and while he's in the Hunter's Nightmare, obviously From wasn't working step-by-step to make it all fit with Ravenloft's conventions.
We're talking about adaptation here, and so we'll need to be a little more concrete and clear in terms of how Yharnam becomes a Domain of Dread.
Laurence as the Darklord needs an act of unmitigated evil to draw him into the Mists (sometimes this act also draws the character's realm with them, or sometimes it creates a mirror of a real realm, or even a wholly new realm. In fact, the original Darklord's domain, Barovia, might well exist in a non-nightmarish form on some Prime Material World, with the OG Ravenloft setting possibly being a nightmarish facsimile of the real thing).
I think there are two potential moments to draw from here.
If we interpret that the "Holy Medium" the Byrgenwerth scholars discovered in the ancient Labyrinth was, in fact, the unborn child, Mergo, and that the healing blood was harvested from Queen Yharnam's womb (it would make sense that some body fluid nourishing a growing demigod fetus could have extraordinary healing properties). Laurence's decision to do this, robbing Yharnam of her child very much against her will and perhaps prolonging its life via the Nightmare of Mensis to keep drawing the healing blood off of it would be a profoundly evil act. (The fact that the blood also warps those who use it into werewolf-like beasts can also be explained by the fact that it was never meant to heal humans.)
However, it was this blood that allowed the Healing Church to be established in the first place. And while you could imagine the whole church only really coming about in the Domain of Dread, I don't know if I love that version of it.
Another possibility is the burning of Old Yharnam.
So, Old Yharnam - which was probably a small town or village before the big city grew around it - is, currently, filled with Beasts - there's just two humans left who have decided to guard it against other humans, reasoning that the Beasts here are just innocents who were turned and don't deserve to be shredded by Hunters.
But the history here is that a plague ravaged the town, and the Healing Church arrived to intervene, and I believe this was the first place they administered blood healing on a mass scale. No coincidence, then, that this was also where the Curse of the Beast first emerged (something the Church also tried to cover up). The thing is, the "plague" as it was called, was also treatable via Antitdotes - pills that cured poison, not infections. The plague was thus probably an intentional poisoning of the populace. And that poisoning was probably committed by the nascent Healing Church in order to give them an excuse to experiment on the people of Old Yharnam.
When the Beast Plague began to spread, they wound up burning Old Yharnam, killing tons of people, but also conveniently covering up the Church's crimes.
I think Laurence ordered the burning of Old Yharnam. And I think the fact that, in the Hunter's Nightmare, he is now rendered as an ever-burning Cleric Beast. This massacre of the Beasts of Old Yharnam is what started off the endless cycle of Hunts. And I think that this could be Laurence's point of no return. Up until this point, he's maybe done things for the Greater Good, but when confronted with the utter failure of his visions of miraculous healing and respect via piety, when it became clear that there was nothing but madness and horror to come from his Healing Church, he decided to murder hundreds of people so that he could keep going.
What is his torment, then?
Well, I think Laurence, being a Byrgenwerth scholar, wanted to pursue humanity's evolution, and thought the Healing Blood was the way to do this - to kind of hijack the Pthumerians' communion with a Great One (which one I think being open to interpretation - the Moon Presence? Oedon?) - but every effort to move forward and push humanity further has led instead to devolution, with people turning into beasts, not higher forms.
Practically speaking, I think Laurence is constantly losing his grasp on his lofty ambitions, but in being transformed into a beast, he's dangerous because of his raw, physical strength.
And if we look at the rest of the setting, we can see the way that the other dangers are reflections of Laurence. The Hunters are the embodiment of his violent sins, the Scholars embodiments of his unrestrained ambitions, the Clerics embodiments of his need for control.
While I think you could even spin it such that the Great Ones were actually the Dark Powers all along, I actually like that in Bloodborne, the Great Ones are actually benevolent in spirit - they're just so alien that their mere presence warps things in horrific ways (perhaps even when they "try to help.")
I don't think you'd have to do much to run a Ravenloft campaign in Yharnam - much as Barovia has a lot of different elements that aren't obviously tied to Strahd, but ultimately are, you can do likewise with Yharnam.
The timeline of Bloodborne is a little vague - you could certainly interpret the existence of Cainhurst Castle as pre-dating the Healing Church, for example, which is thought to be less than a century old, while Cainhurst is supposed to be a few hundred years. But time is also very weird in the Domains of Dread, and so I could imagine that people in Yharnam proper treat Laurence as a current ruling figure, but that the stolen blood that created the Cainhurst Vilebloods was stolen from the Church (and I don't know if that's even directly contradicted by the game).
In Ravenloft, you don't need to make your Darklord the "final boss" of your campaign - indeed, a lot of them have very unimpressive stat blocks that a low-level party would have an easy time defeating. Laurence would probably be tough in his beast form - I'd probably homebrew some changes to the Loup-Garou to give it fire abilities - but you could easily have a campaign focusing on taking down the School of Mensis or the Choir or dealing with some eldritch monstrosity (like Ebrietas).
The key would be to hit the notes of Gothic Horror (werewolves, ghosts, maybe vampires from Cainhurst if you want to retcon the massacre by the Executioners, witches in Hemwick, and grim inquisitors) as well as cosmic horror (all the alien monsters in the later parts of the game, the transformed scholars, and the weird shifting realities).
One thing I'd really encourage as a sort of "special system" to use in this domain is Insight. Obviously, you might not want to call it that specifically, given that that has a very specific and different meaning in D&D, but something like "Sight" or "Truth" or "Eldritch Truth" or "Awareness." I think there would be a positive and a negative associated with Insight, and probably have it be something players are gaining at different rates. Something like a successful Intelligence check could give you a point of "Awareness."
The main way I'd want it to work is that, as you gain Awareness, you can hit points where you'll see things others can't - starting with small things like hidden runes or clues - maybe you need 3 Awareness to see a button that can be used to open a treasure chest. And then, at higher levels, you start to see the sources of certain enemy's powers - like the runic items on some of the new Giants out of Bigby's. But then, for each point of Awareness (or maybe every few points,) you get penalties as well - say, to Wisdom saving throws (maybe all Intelligence, Wisdom, and Charisma saving throws?)
Ooh, and maybe there are magic items that you can't use (or don't work at full power) if you don't have enough Awareness. So, for example, your Arcane Saw Cleaver, which could be a re-skinned Battleaxe, does an extra 1d8 force damage if you have at least 5 Awareness. But maybe at 5 you'd also be taking a -2 to Wisdom saving throws.
I like the idea that delving deeper into the Eldritch Truth makes you more vulnerable but also more powerful. I can tell you my Great Old One Warlock would be happy to amass as much as possible!
Anyway, I think this could be a lot of fun, and almost want to run a limited campaign in it.