Mentioned in Van Richten as one of the "other domains," the Shadowlands is a realm of chivalric knights and an evil sword called Ebonbane.
We often talk about D&D as having a default tone of "medieval fantasy," but the truth is that there's a lot of haziness around what we mean by that. The Middle Ages, after all, were approximately a thousand years (I'm sure that some historians would have differing opinions, but I generally think of it as lasting between the fall of the Western Roman Empire and the discovery by Europeans of the Americas (not really counting the Vikings).
I think a lot of settings feel a little more like the Renaissance era, and certainly a lot of us (myself certainly included) like to mix in some anachronistically modern ideas that may not have cropped up until even the Industrial Revolution or later (Eberron, for instance, is meant to feel like the 1930s with all the Pulp Adventure stories told in that time).
But if we are to consider the foundations of the fantasy genre, while we certainly cannot forget things like Greek Myth, I'd say one of the most central influences is Arthurian Legend. There's something about the story of King Arthur and his Knights of the Round Table that feels really fundamental to how we think of fantasy.
And the Shadowlands deliberately harkens to this particular bit of legend: Ebonbane is effectively the inverted, shadow archetype of Excalibur.
Now, there's a certain aesthetic that I associate with this kind of Arthurian fantasy - I can't say precisely where it comes from, but one of deep green forests, the ground covered with moss, the woods dark with a thick canopy, and a lone knight in armor on horseback venturing through it, bravely facing whatever dangers might lurk there. I don't know precisely what the source of it is - I have a vague sense of it being linked to movies from the 60s through the 80s, though it might also be partially inspired by Disney's Sleeping Beauty, which borrowed some of its aesthetics from Medieval illustrated manuscripts.
Ravenloft covers a lot of different vibes, but it tends toward Gothic Horror. The funny thing is that in some ways, the Gothic Aesthetic was built on those in the 19th century looking back on the ruins of centuries long past with a mix of fear and nostalgia. The novel Dracula takes place contemporaneously in the 1890s, at a period in which electrification and industrialization were in full swing, but Dracula emerges in that future-oriented world as this strange and forgotten relic of a brutal medieval past, living in a drafty castle, enriched by the spoils of ancient battles and wars.
What the Shadowlands presents us with is an opportunity to really live in that medieval world. But while a classic Arthurian realm would have brave knights who do oppose the evil of that domain, the Ravenloft twist is that the greatest potential source for good is the fanatical inquisitors under the command of Elena Faith-hold. Elena is not the Darklord of the domain, though should its lord fall, she'd likely be next in line.
Instead, the lord of the Shadowlands is Ebonbane, a sentient sword.
Unlike most Darklords, who began as mortals and then had a fall from grace, Ebonbane is pure evil. Forged by cultists who opposed the heroic knight Kateri Shadowborn, Ebonbane was made out of the essences of all the evils she had defeated, and ultimately it slew her after she had gotten old and slower, then possessed her body in the hopes of doing more evil, but was then swallowed by the Mists.
Incidentally, I actually think that this makes Ebonbane a little similar to Exdeath from Final Fantasy V - in that story, long ago, a whole bunch of evil spirits were sealed within a tree. Over time, they coalesced into a single ego, and then that tree kind of grew a tangle of thorns and branches into a dark suit of armor. You spend much of the game assuming that Exdeath is some kind of evil knight.
The problem for Ebonbane, though, is that outside of Shadowborn Manor, it cannot act unless it is carried by another.
There's a lot to work with here:
First off, you can definitely fake out your players with who the actual villain of a campaign is. Ebonbane is considered a Construct while in the Manor, and can fly around like a (far more powerful) animated sword, but to go elsewhere, it needs a wielder. This can include a corpse that the sword possesses, meaning that you could easily have a figure that looks for all intents and purposes like a Death Knight of some sort, wreaking havoc across the land, burning villages to the ground, etc.
What's interesting is that the corpse isn't even considered undead: it's basically an object that Ebonbane is wielding by having the corpse wield it. A subtle hint that the party might have that there's something odd going on is that a Divine Sense might show that that moldering corpse that's walking around doesn't actually show up as undead.
The party might even fell this figure (which would be a fight against Ebonbane, using the sword's statistics) and try to figure out what to do with the evil sword the villain was carrying.
Now, the mechanics here require a little DM finesse - technically it's still the sword that the players would need to attack. But I'd extend this possession to basically let the body stand in for Ebonbane. Slaying Ebonbane makes it a +3 Longsword until it revives through Darklord Restoration, but if you don't want 1d10 weeks to pass before the Darklord becomes a threat again, I might rule that while it's possessing a corpse, maybe it drops its possession when the 50 temp HP goes away or, if that's too soon, maybe when Ebonbane gets bloodied. Presumably not being in the Manor, it would drop to the ground and appear to just be a powerful magic item.
Ok, now, how do you convince your players to try to wield Ebonbane?
Well, I think this is where Elena Faith-hold can play an important role:
In the Shadowlands, the Circle is effectively the equivalent of the Knights of the Round Table, an order of chivalric champions who are supposed to defend the good and fight the wicked. There's literally a big round table in Shadowborn Manor. The symbolism of the round table, of course, is that the order is one of equals. Thus, while Elena is a very powerful and influential member of the order, she's not its "leader" because no such role exists.
Still, she has taken the "fight the wicked" half of her charge far more seriously than the "defend the good," and even does things like feeding innocents to a dracolich to prevent the common folk from learning that the evil dragon she'd slain isn't actually gone.
In other words, she's a hatable villain that could easily feel like a major threat to the party and play the role of primary antagonist for the better part of an adventure. And it is that kind of evil that Ebonbane might present itself as the key to defeating.
So, Ebonbane can act both as a major boss fight as well as a powerful magic item. It is, also, an actual appropriate candidate for a real Hexblade patron for a Warlock (though we'll have to see if the new version in Arcana Unleashed is actually appealing to anyone given its dismal showing in Unearthed Arcana).
While Elena Faith-hold is a monstrous hypocrite, the Circle itself as a faction is still I think meant to largely be a good one. PCs might find allies amongst the knights (or even be members). But given that this is Ravenloft, these knights will likely meet tragic ends. A former ally might die trying to warn the party of the evils of Ebonbane, only for them to show up as an animated corpse wielding the blade.
Anyway, I think this domain could be an interesting aesthetic change for a general Ravenloft campaign, and could let you go for a very classical "knights & dragons" fantasy vibe that we honestly don't see very much in D&D.