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.
In previous editions, Ravenloft was more connected than it is in 5e lore. You could travel from one domain to another just as easily as you could travel from Baldur's Gate to Waterdeep in the Forgotten Realms, or from Sandpoint to Magnimar on Golarion, unless a domain's borders had been closed by its Darklord (in which case you got lost in the mists and found yourself, usually, back where you started.)
The fact that different domains are at different cultural levels was just a regional quirk, which I think tends to hold true of certainly Golarion today. A player character might travel from a very classic fantasy town to a city filled with steampunk wonderments, and it doesn't break immersion because fantasy has proven to be a very flexible genre. Fantasy horror is equally flexible.
Back then, Azalin Rex was the darklord of Darkon, and he was a mutually despised enemy of Strahd von Zarovich in nearby Barovia. This minor subplot probably felt awkward in 5e's version of Ravenloft, where each domain doesn't quite seem to know that other domains exist. The 5e Ravenloft consists of little "bubbles" within the mists, with everyone inside each bubble believing simultaneously that their world is infinite and yet also that they are, for whatever reason, trapped within their local region.
So in this book, Azalin Rex is missing. He wasn't just forgotten, though, and it's stated very clearly that he once existed, but is now missing. There are hints about what may have happened to him, but nothing is certain. Darkon has no darklord currently, although there are several characters attempting to take control. In a sense, Darkon hasn't got one darklord but three.
As with other domains, I'm struggling to understand the significance of the darklords in 5e. They don't get stat blocks, and the stat blocks that get referenced for them don't tend to be particularly powerful, and yet Van Richten's Guide tells me to feature them in the adventure. What's to keep players from going from one domain to the other, systematically slaughtering the darklord? It wouldn't be that difficult.
Maybe Darkon is an example of why that's not a great idea. You get rid of one darklord, and three rise to takes its place.
In 3e Ravenloft, Darkon was a dark fantasy domain, with an evil lich, looming mostly unseen, in the background as its ruler. It consisted of several cities, mysterious forests, and murky swamps. Azalin Rex ruled absolutely, and his enforcers were the Kargat, a secret police force of vampires, ghosts, and lycanthropes.
In 5e, Darkon is a disaster. With its ruler missing, the domain is being consumed by something called The Shroud, a particularly destructive form of mist. Life goes on within the cities in Darkon, as most of the population lives in denial about their world being consumed around them.
The map of Darkon is essentially the 3e map after somebody took an eraser to it. There are sections arbitrarily missing, presumably because the mists or The Shroud have over taken it. This is a little confusing to me, because if the missing parts of the map are The Shroud, then how are players meant to travel from, say, Rivalis to Il Aluk without being erased? I guess it must be mists, but that seems like a rather severe way to guarantee that No Adventure happens between any two cities. Definitely an odd choice.
During his reign, Azalin Rex's home, Castle Avernus, was the site of masquerade balls and official events. Azalin Rex didn't attend these events himself, but if you were of a certain social standing, you could gain an audience with Azalin Rex in his throne room.
Now that Azalin Rex is gone, the castle is abandoned to a non-specific extent. This, for me, is an obvious invitation to either build your own dungeon with a built-in backstory, or to drop-in that random dungeon you bought but haven't had the chance to run yet. Who knows, maybe even a little section cut out of Black Monastery or Rappan Athuk.
The more I read about each domain, the more I become convinced that this book expects each domain to represent a one-shot adventure. I'm almost certain that the authors expect player characters to either join the Carnival and go through the domains sequentially. At the beginning of each "episode", you arrive in a new domain. You get caught up in a local conflict. You solve the problem. And then at the end of the "episode", you pack up and move on through the mists.
An alternative is to find mist talismans at the end of each "episode", and then move on because some NPC tells you to go or another NPC beckons you to their home domain.
The genre listed for Darkon is Dark fantasy and disaster horror, and the plot for Darkon reflects that. It's clear from the plot tables that you're meant to "escape" Darkon. It's being consumed, but you have to find a really really important MacGuffin, so your party goes in, has an adventure, and just as you have your hands on the thing you need, the Kargat and the Shroud and one or three of the darklords show up, and they want you either dead or stopped or both.
I think it would make for an invigorating adventure, and even though I keep thinking about how there's so much to Darkon that's not covered in the 6 pages of this book, I continue to acknowledge that realistically all you need in a domain of dread is one adventure. The domains are disposable, because as with all fantasy, they can be reset for the next campaign! Destroy Darkon, and try to destroy your player characters along with it. This is the kind of thing that makes gaming fun.