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 the controversial work Struggle and Victory in the Animal Kingdom, [fictional] naturalist Inard Milhavet wrote that the black rat was the most successful of all beasts. Maybe Milhavet had a point, because rats dominate Richemulot. In its 3.5 version, this domain had a wererat problem. In the 5e version, Richemulot is literally a plagued by rats, spreading a disease called "The Gnaws." Wererats are immune to the Gnawing Plague, and so they've come to view Richemulot as a haven for filth and evil.
On the surface, Richemulot is a nice place, though. It's a got pastoral countrysides, bustling and industrious cities, and the glamourous Jacqueline Renier, a firm and capable leader who also happens to be...a darklord.
Jacqueline Renier is an aristocrat who inherited the domain of Richemulot from her grandfather Claude Renier. Claude was cruel and treacherous, and his power in Richemulot ended when Jacqueline murdered him so she could inherit his power before her twin sister Louise. She didn't murder her twin, interestingly, but I guess Louise is happy for her sister to rule because there's no indication of a power struggle (yet). This isn't an evil twin and good twin situation, either. Both sisters, like the entire Renier line, are evil wererats, with Jacqueline's rat form being an especially vile dire rat.
The Gnawing Plague is the strongest element of Richemulot as a setting, and there are disease rules in effect here. The plague undulates between Stage One (Threat) and Stage Four (Pestilence) at the Dungeon Master's whim or the roll of a d20. The chance for infection bizarrely goes down at Stage Four (1 in 6 chance, although with a very high DC), but the chance of wererat or rat combat goes up. During other stages, Constitution rolls to avoid the disease are frequent.
The disease's mechanical effects are:
That feels pretty fair, and relatively simple to manage mechanically.
Obviously a plague in the real modern world would be relatively simple to eradicate. The population would wear masks, and get a vaccine in the interest of protecting themselves and their fellow human beings. The disease would be starved of hosts and die off. But in the pure fantasy world of Ravenloft, things are not so simple. In Richemulot, the despotic leader is herself a wererat (though she'll never admit to it) and probably encourages poor health. Heck, she may even encourage charlatans among the population to claim that the disease doesn't exist or that reports of its deadly effects are exagerrated. She might use the local churches to make claims that the only true healing available for the disease is faith, and that masks were a sign of a lack of faith, and an affront to the noble Renier family.
None of this would happen it the real world, obviously. We live in the 21st century, with amazingly advanced scientific knowledge, readily available education, mass media and the Internet for the spread of true and accurate information. But in the fantasy world of D&D, these things may as well be as magic is to us in the real world. And so Richemulot is truly a horrifying setting, and thankfully a stark contrast to the world you and I live in.
Richemulot doesn't necessarily have a strong plot suggestion, but I think it's one of those domains that can serve as a strong setting for any-old-adventure. Send your players into Richemulot for any MacGuffin, and force them to battle the invisible plague (and the visible plague of wererats, once they make themselves known.)