I believe the second Darklord ever introduced to D&D (though Azalin might have also shown up in that module,) Lord Wilfred Godefroy is the ghost of a murderous megalomaniac who continues in death to want dominion over others.
He's also one of the lower-CR Darklords in Horrors Within, at only CR 6 (I think the only lower ones are Ivana Boritsi and Ivan Delisnya, who are both 5). We actually don't have a lot of legendary creatures with a CR lower than 10, so this is kind of an exciting opportunity.
As the Darklord of Mordent, Godefroy is the hauntingest ghost of the ghost domain.
Ghost stories have to tread a fine line: they live in the uncanny, the liminal space between what is a familiar human (or humanoid at least in a fantasy world like those of D&D) person and something monstrous and terrifying.
In the past year or so I watched through the first three Netflix series that Mike Flanagan did. While the vampire one, Midnight Mass, was my favorite, the first two, The Haunting of Hill House and The Haunting of Bly Manor, which are inspired by the works of Shirley Jackson and Henry James, respectively, deal with haunted houses.
Both are houses that, like the House on Gryphon Hill, are haunted by not one but many ghosts. In the case of Hill House, at least in Flagan's version, it's the house itself that is the monster, though some of the ghosts there further its evil by drawing other souls to become trapped within it. In Bly Manor, there's a clearer genesis for the haunting, but again, it becomes something of a snowballing tragedy, as the actions of selfish and/or desperate, or even just compulsive spirits draw other souls to become trapped.
Another important point, though, to these is the gradual emergence of the horror. There's an amazing reveal in Flanagan's Hill House (I make the distinction because the Jackson novel has a very different plot) about how the house has been tricking the family all along that I'm going to avoid spoiling here (might touch on it later past a spoiler break) that I think can be a really powerful tool in a story like this.
But I think that even if you want to have a villain (especially Darklords) show up early on in a campaign, these appearances don't need to be full confrontations. Especially in a ghost story, a brief glimpse of a ghost is the right way to introduce them.
In Ravenloft, the Domains reflect and are kind of an aspect of the Darklord, and I think we can definitely pull this with Mordent and specifically the House on Gryphon Hill.
Here's my proposal:
The party arrives in Mordent at level 1, or level 3 at max - this should be the beginning of a Ravenloft campaign if not just a limited tier 1 adventure. They are welcomed by the locals, who are eager to have some capable adventurers around to help with local problems. They are given a house to act as their headquarters. But the Mists warp their perception of it:
While the party is maybe even warned not to go to the House on Gryphon Hill, and the house they are presented with is on the other side of Mordenshire or even potentially in a different town, and has a different name, the truth is that they're getting turned around: the house they are in is the House on Gryphon Hill.
It's separated enough from town that when they visit, it's not obvious that they're coming from it, but characters will have strange moments where they find themselves on the road to Gryphon Hill, periodically.
The house, as it appears to them, is well-furnished and kept up, and they have a handful of short, spooky quests while coming back to the false safety of the house. But after several days staying there, in the middle of the night, a random party member wakes up to use the restroom, and as they walk down the hall, they suddenly feel a presence behind them. They're subjected to Godefroy's Possessive Aura, and if they fail the save, they just stand there until morning comes and the party finds them as if they were sleepwalking. If they succeed, they turn around and glimpse Lord Godefroy staring at them, looking furious, before he vanishes.
Now, yeah, these aren't part of his stat block precisely - the Possessive Aura only charms a creature for a minute and reduces their speed to 0. And despite being a Ghost, Godefroy weirdly doesn't have the ability to slip into the Ethereal Plane, but I'd allow it at least in a non-combat situation.
While the party is likely to be genre-savy enough to freaking move out of the house at this point, there will probably be some compelling reasons for them to stay.
You could run this as a single horror-movie scenario that all takes place over the course of a single night, the doors locking the party into the house. But I'd be more interested in a slow-burn. Maybe other potential solutions - staying in inns, camping out on the moors - wind up being untenable. Or, perhaps, the party does relocate but needs to go back to the house. Perhaps an ally, like one of the Weathermay-Foxgrove twins, was seen entering the house.
Now, a good haunted house has a good cast of ghosts. I'd build the adventure around a number of distinct ghost characters. Especially if the party is likely to fight the ghosts, try to use different stat blocks: Shadows, Specters, Ghosts, Wraiths, etc., and even potentially non-incorporeal undead like Wights or Revenants.
Each spirit that is highlighted in the adventure should have their own distinct backstory, some distinctive visual identifier and MO. For example, in Flanagan's Hill House, there's a man in a bowler hat who haunts the house, and he follows one of the siblings who survived the house back when they briefly lived there in the 90s, always floating behind him as a metaphor for the substance addiction he now struggles with.
The spirits need not be malevolent, but their actions might terrify or even do harm to those with whom they interact. A terrified murder-victim might lash out in what they think is self-defense, and might not be able to calm down until they are assured that nothing can hurt them anymore. A rakish thief might violently defend the treasure that they sought to pilfer until that treasure is destroyed or removed from the house. None will go away simply by defeating them in combat - their stories need to be resolved in order to allow them to find peace and stop threatening the inhabitants of the house. A t
Now, Godefroy's role in this is that he's going to actively work against the party in their efforts to resolve these stories. Ghosts tend to be fixated and obsessed with particular desires, like the rest of their mind has faded and left only this compulsion. Godefroy probably understands his ghostly nature a little better than others, but the fixation remains on control, and so he doesn't want to lose the power he feels he gets by keeping these ghosts trapped and agonized.
He can certainly attack the party, but he might also attack the other spirits that the party is trying to soothe.
I think Godefroy is probably pretty aggressive. Sticking to narrow rooms and hallways, he can use his incorporeal movement to escape if his HP gets low, in case you don't want to risk relying on his Darklord regeneration to get him back. There are probably a lot of locked doors (even doors that might become locked) that could give him greater time to escape by entering a locked room, then descending through the floor, and eventually coming somewhere the party couldn't possibly pursue him.
Also, I think he'd likely employ some hit-and-run tactics. If the party is helping some Poltergeist resolve their issues, he might come in, blast the spirit with his Hunting Rifle, and then slip away.
Ghost stories are often told as the aftermath of tragedies, and Godefroy has some tools to encourage such tragic events - if he can get someone with his Possessive Aura (which admittedly isn't terribly hard to get out from if you don't mind drawing Opportunity Attacks) he can then redirect attacks against him at the charmed creature - getting PCs to damage each other could create divisions or at least guilt.
At 2900 xp in his domain (though this only adds an extra Legendary Resistance) he's theoretically a high difficult encounter on his own for a group of 5th level players. I'm honestly a little skeptical about that, as he only has 72 HP and deals 9 or 10 damage with his two attacks. He does, of course, get some more damage out of his legendary actions.
I do generally think that I'd have this whole plot primarily in tier 1, maybe letting the party do a couple adventures at levels 1 and 2 before the haunting of their headquarters becomes clear.
And then, I think, the first true full-on fight with Godefroy is where the truth of where they are gets revealed.
If you want to move on, either ending the adventure or moving on to another domain of dread, you can do that. If you want to stick around in Mordent, Godefroy is going to need some minions when he appears again.