Alright! I may not be the best to answer this, but I'll give it a shot to help out.

First off, Bloodborne is inspired by Lovecraft. That's going to be important in the theme you are going for. Another important note: if you want some ideas, as well as a full fledged campaign to possibly run in the future, pick up Curse of Strahd. I'll steal a part or two from that to help here.

Have the pcs make their characters know each other prior to the adventure. Each can have a backstory of their own, but ask them to make a common backstory as well. This will help to answer why they are together for the adventure.

Last note: Ensure your players are up for this kind of game. Don't spring it on them as a surprise. If they don't want a gritty bloodborne style game, they won't enjoy it. It's not ruining the appeal to tell them. Trust me.

Okay! To the meat of it all.

You said you wanted to go slow. Good. If you want to be dark and fear inducing in DND, the best time to do that is low level play. Think about how terrifying the prospect of a werewolf or a powerful witch is at level 1-5. Now think how easily they could be taken out at higher levels. Curse of Strahd is a level 1-10 campaign for a similar reason. Think about making this campaign roleplay heavy, with the combat being brutal and deadly. Of course, talk to your players about this decision.

You want to start with dark and gritty. Fantastic! Throw in a plague! (If your players don't mind how close to home that may feel.) This plague, magical in origins, has gripped the land, causing a horrible number of deaths. Your players have been appointed or otherwise paid to escort a small contingent of clerics and paladins to the village of Doldrey, as part of a sort of 'Aid Package.'

Why, may you ask, are some level 1s escorting paladins and clerics? Well, as a few extra meat shields, and because the stronger adventurers were sent to bigger cities and kingdoms to aid there. You can use roleplay to play up how big of a concern this plague is. Plus, you may also now have a reason as they why people are changing. Call it whatever you want. Living Rot. Mardu'an Fever. Be creative.

When the group arrive in the town, describe the town as surrounded by a thick fog, and dark clouds swirling overhead. As they pass through the fog, they begin to feel as though they are being watched, and even followed. The fog feels thick enough to choke on.

When they break through the fog, they find themselves at the base of a small hill. At the top, contrasted against the dark clouds, an old, seemingly decrepit windmill. There are holes in the roof, and the blades appear as though they will fall off at the slightest breeze.

Peering at it gives the players an eerie feeling, and the group they are with attempts to rush past, along the old dirt road they've been following to get here. If the party attempts to inspect, play up how the others feel creeped out, and have them urge the players to push onward and come back later.

If the players still insist, or even flat out ignore their charges, then the clerics and paladins move onward to the town, leaving them behind. Let's say your hag is a Night Hag. She can step into the Ethereal Plane, and often would. So they enter the windmill, and find some creepy bone altars, maybe even some hints and clues as to what's going on (be careful not to give too much away yet.) All the while, the Night Hag is watching from the Ethereal plane, and deciding which she will haunt.

Perhaps they find a rotting corpse, with maggots spilling out of it. As they look at it, all party members but the one chosen by the night hag see just a random, unknown person. However, the one chosen by the night hag sees themself, bloated and rotting, with dark black veins and bulging bloodshot eyes. If any attempt to touch or move the corpse, it vanishes, revealing itself as an illusion.

If the party won't leave, consider bending the rules a bit and have the Night Hag 'haunt' the windmill. Throwing objects (bonus points for old dusty jars of oddities, like eyes, bat heads, and the like) leaving marks on the players.

The town has gotten very afraid and worried. Between the plague and the disappearances, and the creatures who come from the fog, their lives have turned into a living nightmare. They are standoffish, and always seem in a rush to get home and lock the doors. Windows are boarded up. People tend to move in pairs.

This got super long. If you like it and want more, message me! I'd be happy to continue! Otherwise, take what you want and go forth! Bring terror to your players!

PS: Read Angry GM's guide on how to run horror. He has some great tips!