Forlorn.
Our narrator is not fond of the place, calling it a bleak, miserable, valueless wasteland - there's lots of things out there that our narrator doesn't like very much, but in this particular case it's hard not to agree. Forlorn is largely forested hills, black sucking marsh, and craggy highlands, with a few ruins of varying ages serving as a reminder of when the place was once actually civilised and populated. The forests are largely rotting, fungus and moulds, predatory plants and vermin proliferate, while the timbers are largely foul and valueless except for small patches which the resident druids keep somewhat intact.
Speaking of the druids - I feel the compulsion to complain about population numbers here again. The population of Forlorn itself is listed as around 2000, of which 94% are goblyns. That's around 120 humans (Forfarians, they call themselves, after the lan's old name before it was taken by the Mists), plus about another 300 in the village of Forfarmax just across the Hazlan border, and a few hundred more in Immol across the Barovian border. And it's been that way for a couple of hundred years since the domain got dragged into Ravenloft. These just aren't population numbers that can sustain themselves without significant inbreeding, let alone fight continual wars against goblyns and have a duel-to-the-death tradition, while still sustaining multiple factions of druids, several noble or high-ranking extended clan families etc etc. I expect this issue to pop up in future reads as well, and I promise not to whinge about it every time, but jeez, it would have been nice if someone did some maths. Not sure I'd be 100% comfortable multiplying the population of EVERY domain by 10 as someone above suggested, but Forlorn is certainly a place that could be improved by this.
We then get several pages of Forfarian history. Basically, it all started to go wrong when a hero came back from the wars infected with vampirism, and was hunted down and destroyed but not before his princess fiance bore his child regardless. The princess was lynched by an angry mob and the child taken away by druids, but all too late, the place was cursed, and it was a downward spiral of hauntings, mysterious deaths, cruel lords and ruling families getting slaughtered wholesale by accident, chance, and design until eventually it all got too much and overnight the whole land was dragged into the mists, the forests became tainted and unwholesome, and a large majority of the population were turned into face-eating goblyns, which left everyone else fleeing for the borders for feat of getting their faces eaten. Or that's the truth as far as the average Forfarian knows, in reality ... spoiler alert! ... it's the half-vampire child (the Gazetteer refers to him as a vampyre, I guess in 5e Ravenloft he's more likely to be termed a dhampir) who's been largely the problem all along, and through an extremely complicated sequence of events involving misunderstandings, curses, ghosts and false identities, is now the Darklord.
Currently the main population of Forlorn is goblyns, who are basically your standard nasty sneaky torture-y D&D humanoid bad guy, with a penchant for eating people and a recently acquired habit of cutting down the forests and leaving behind a stinking morass of filth and ashy mud on behalf of their mysterious overlord (three guesses who this is and the last two don't count). The wrinkle I found most interesting (and under-used) here is that they still adhere to a debased version of Forfarian traditions, dress, language etc from their human ancestry. They even worship a god from the Forfarian (ie, Celtic) pantheon. An evil one, but still.
Forfarian humans are Scots. Sure, they use the prefect ap- instead of mac- for their clan names, but they're highlanders straight from Hollywood central casting. Haggis, caber-tossing, kilts and sporrans - even bagpipes make an appearance. And then of course there's the mysterious lake monster, just cos if you're on a good cliche, stick to it. As mentioned above, they do have their own pantheon, who are largely neglected in favour of druidism these days, but they get a very brief writeup in the appendices. Ravenloft deities and pantheons are a profound mess, to be honest, and Forlorn is one of the worse examples. We get another example of the no-demihuman domain, and of the domain where arcane magic is frowned upon. Which is definitely starting to be a theme.
Forlorn was, I believe, the third domain ever to appear in Ravenloft materials, and to some degree it shows. The whole place is built around the Darklord. If you're not fighting the Darklord, there's not a lot to do here - even if you show up at low level to rescue a captive from goblyns (for instance) - even the goblyns work for the darklord. This is the quintessential example of a domain designed as a weekend in Hell adventure. Fortunately, I think it'd be a GOOD one, if that's the way your taste runs. Castle Tristenoira, with its many and varied ghosts and curses and habit of shifting you back and forward in time to pivotal moments in Forfarian history, would be an amazing haunted house setting. I don't own the Castles Forlorn supplement, but i got to read it a long time ago and I think that's largely the idea. If you want an epic Ravenloft haunted house mystery to solve, Forlorn might just be for you.
This Darklord-centricity seems to have given the Gazetteer writers, with their apparent philosophy of making all the domains part of a living functional world, some problems. You could come up with reasons for PCs to come to Forlorn if you really wanted to. Sourcing poisons from the tainted forest would be one possibility, but seriously, Borca is RIGHT THERE almost certainly much more accessible, and if there's a poison you can't get there, it's probably not worth having. The authors seems to have tried their best to add other unrelated challenges or places of interest here - Castle Forfarmax is the little haunted castle you can sharpen you teeth on (I believe it's documented in Children of the Night: Ghosts, which I don't own) before trying your luck in Tristenoira. There's an evil undead treant out there who hates druids and goblyns equally, he was killed ages ago and used for lumber, but he has since reconstituted himself by building a new body from all the furniture etc that his timber was used to construct, which is a ... memorable ... mental image. There's a distrusted clan of wizards who have - so far - always defended Forfarmax from goblyn raids. And you've got the lake monster, which doesn't seem to do anything interesting or serve any purpose other than revel in the Scottish cliches and be even scarier than regular lake monsters due to being undead, apparently, and provide Castles Forlorn with some great cover art. There's a few mentions of fey creatures as well, because Scotland, but they're believed to largely gone from the domain, except possibly for the druid who wants to summon the Wild Hunt to hunt the Darklord (and while this might not quite be the absolute worst idea anyone's ever had in Ravenloft, if it's not then that's only because of the stiff competition!)
What you do with Forlorn in 5e - well, it depends what you want the domain to be. I do think Tristenoira would be an amazingly atmospheric and uncanny long-run adventure site, even with its overly-complicated Darklord, so you could quite possibly hone in on that as the focus of the place. If you want it to be a bit more flexible and less Darklord-centric, I think the thing to do would be to bump up the human population a bit, and emphasise the common roots of the humans and goblyns. As a Forfarian, Forlorn's horror is all about slow inexorable loss, of precious things rotting away, and hanging on to the scraps of your identity in the face of its slow obliteration. Even the Darklord is locked alone in his castle surrounded by the ghosts of anyone who ever gave a damn about him and died because of it, and the constant reminders of his murders and failures and how everything he touched he destroyed. I'd give the goblyns a little malicious humanity back. Let them remember the clans they came from originally (the book is not clear on what the people of Forfar did wrong to be arbitrarily turned into face-eating goblyns as part of their ruler's cosmic punishment, but I guess they're called the Dark Powers rather than the Just And Honourable Powers for a reason...). Let them leverage that knowledge - have them call in hoary old favours and debts of honour owed and ancient oaths of clan friendship and hospitality from the human Forfarians. Make the humans choose between abandoning their oaths and most sacred traditions, and accelerating their own free-fall away from what they once were, or possibly letting a face-eating malice-goblyn into their hearth, or doing it favours, and make them do this in the full knowledge that the goblyn WILL betray them. Goblyns don't just want to eat faces (though faces are tasty and delicious...), they want to drag the Forfarians down to their level and then rub their noses in it.
Today the random class picker spat out sorcerer, so our PC for the day is this guy, a seventh son born under an ill-omened dolmen in a haunted glade. Like every Forfarian who lives in Forlorn proper (all 120 of them...) he was raised on the assumption he would become a druid, but eventually the call of the shadow became too insistent to be denied. Alas, I don't have Pro access to Heroforge so i can't put a tartan pattern on his kilt, use your imagination, ok?
Next up, Kartakass, which will be fun because I remember almost nothing about it.
