I picked up Van Richten's Guide to Ravenloft and have been reading it cover to cover. This is my review of the book, chapter by chapter. Chapter 3 covers over 30 domains, so I'm posting about it as I work my way through the different domains.
The domain of Dementlieu, in previous editions, was essentially just France land. It was the part of Ravenloft where you could go to feel like you were fighting vampires and werewolves in France instead of Eastern Europe. Its darklord was Dominic d'Honaire, a dangerously charming man. When you interacted with him, you had to makea DC 19 Will save (Will was a 3e mechanic) to resist his charms, and after three consecutive failed checks, you became Obedient.
I imagine that the authors of 5e Ravenloft felt like that was all pretty generic, because the 5e Dementlieu is far far more interesting. However, the trade-off is the same as with the other domains in this book: you get a really strong story, but just the one. The 5e Dementlieu has, more than any other domain so far, a gripping backstory and exactly one story.
Dominic Dementlieu is out. He presumably never existed, and it's no great loss. He was basically an incubus, and those are a dime a dozen. In his place is Saidra d'Honaire, a dark Cinderella who was ridiculed for most of her life for not being of noble birth. So she murdered her way to nobility, caught the Red Death, and became a wight.
In her undeath, she masquerades as a living human, and rules Dementlieu in blissful tyranny. It's "blissful" because everyone in Dementlieu pretends to be better off than they actually are. The lower class pretend to be middle class, even though at night they have to go digging through the rubbish bins for supplies. The middle class pretends to be upper class, even though they have to literally starve themselves to afford their lifestyle. Anyone who betrays the actual state of their country is literally disintegrated by Saidra d'Honaire during her ghostly midnight patrols.
As with Darkon, Dementlieu has one strong story. Saidra d'Honaire throws a masquerade ball every week, and everyone in Dementlieu longs to attend. From what I can tell, the intent is that player characters arrive in Dementlieu, work to obtain an invitation to the ball, and then attend the ball. The reason, of course, is a MacGuffin and there are tables to help you figure out what that is.
The horror genre provided for this domain is dark fantasy and psychological horror.
I'm happy to see that Dementlieu integrates the Red Death. In previous editions of Ravenloft, the Red Death was a common threat throughout each domain. In fact, there was even a supplement called Masque of the Red Death, which existed explicitly so you could run a D&D game in "Gothic Earth in the 19th century."
The Red Death and Ravenloft are tightly bound, so it's nice to see it mentioned along with Saidra d,Honaire (who glows red in her wight form).
I continue to be mystified, though, by the 5e Raveloft's insistence that each domain is almost impossibly small. It's like the Star Trek TOS episode in which Kirk and the gang visit an Old West town, only to find that there are invisible walls to keep them from leaving. The countryside of Dementlieu, called Chateauxfaux (this name predates the 5e version, but seems oddly appropriate to the new version), is literally fake. There's no countryside. You can see it, but when you start walking toward it, it never gets any closer. I don't think I'd run Dementlieu this way, myself. I think if people wanted to explore the countryside, I'd let them, and I'd drop in side quests as required. I understand that each domain is meant to represent one episode of your D&D game TV show, but for I guess I don't see the point in limiting the domain's potential.
One point of confusion for me is the actual origin of 5e Dementlieu. The book has stated that Dark Powers create domains of dread specifically for a darklord. But the backstory of Saidra d'Honaire makes it seem like Saidra rose to power in Dementlieu. I guess we're supposed to assume that Dementlieu exists within some other realm, and that Saidra's backstory happened there, until her death, at which point presumably the Dark Powers placed her into Ravenloft's Dementlieu?
This isn't the first domain that lacked a clear separation of the domain from its backstory, and I think it exposes a slight bug with transferring Ravenloft from a realm of its own to an other-worldly demiplane. If Ravenloft is a prison created by the Dark Powers, then where did the origin stories that got the darklords into their domains happen?
Nevermind the technical details, though. Saidra d'Honaire has one of the richest backstories of any of the darklords so far. I was enthralled while reading it, and I actually felt sympathy for her. I found the faulty first-world facade uncomfortably realistic (maybe being an expat US American helps), so for me the new Dementlieu is one of the strongest domains so far.