I've been re-reading my copy of Paizo's Rule of Fear, a location-centered splatbook on the nation of Ustalav. Paizo's modus operandi is to give each of their nations a strong "hook" which roots it in genre or location specific tropes. For example, the Land of the Linnorm Kings is the Viking country, Osirion is the Egypt country, etc. Ustalav is the horror fiction country, and needless to say, it's one of my favorites.
So much so that, even though I've been developing this here DARK•HERITAGE setting for a long time, I'm tempted to propose as our next game (that I would run) an E6 3.5 D&D game--played fairly straight in terms of rules--that takes place in a modified version of Ustalav. I'd borrow quite a bit directly from the setting book, with really only kinda minor changes. The orc nation to the west would become a breeding ground of beastmen (a la Warhammer), I'd also borrow skaven and put them in the dark corners of much of the world, I'd borrow a bunch of stuff from the Iron Kingdoms setting (which as a d20 setting was quite a bit more horror and dark fantasy oriented than the later Warmachine and Hordes books made out. The success of those games and their style fundamentally changed the tone of the setting, and not for the better.) From Iron Kingdoms, I think a lot of the Circle Orboros faction is tailor-made for a horror campaign. And of course Cryx is as well, although they're a bit over-the-top, and have had the unfortunate habit of sweeping up lots of independent evils, especially of the undead or overtly horror style, under its wings in a way that's not satisfying to a fan of the roleplaying setting rather than the miniatures wargames. Many of the werewolf specific areas of Ustalav can instead of thick with Tharn, for instance, occasionally with Blackclad leaders. The Lord of the Feast is just plain terrifying, as are many of the other wilderness horrors detailed in the Monsternomicon books.
The Vampire Counts army from Warhammer has also evolved considerably in recent years, and fits hand and glove with Ustalav. I really like the variant vampires, and the devolved monstrous vargheists, varghulfs, the terrorgheist and more. Chaos in general is really dark and scary in Warhammer (except when it turns tongue in cheek, which thankfully it's been a little careful to avoid lately) and needs to be borrowed from a lot. After all, the Worldwound is not unlike the Chaos Wastes, and the Worldwound is supposed to share a border with Ustalav too.
Putting all of this into a kitbashers pot, and making sure not to drift too far from the original Ustalavic framework (which I should point out, is also surprisingly humanocentric for a D&D setting kingdom) actually makes for a quick and dirty setting that I actually think I'd really like to explore as a D&D player or GM either one.