Bluetspur: Your One-Stop Shop for Alien Abduction and Cosmic Horror

(Based on info from The Digital Dungeonmaster's page-through video)

 One of the long-established Ravenloft domains, Bluetspur is quite different from what you'd find in other domains. The domains of Ravenloft are filled with terror and horror, but most are at least somewhat livable. People are born, grow up, and die (not always even violently!) in these domains and can live somewhat normal lives.

Not... so much in Bluetspur.

The Darklord of Bluetspur is The God-Brain, an Ilithid Elder Brain that consumed others of its kind to grow in power and knowledge. When it began to deteriorate from some bizarre affliction, the other Ilithid hives psionically banished it from existence, seemingly destroying it, though in fact the God-Brain was taken by the Dark Powers.

Bluetspur is Ravenloft's purest Cosmic Horror domain, and does not have any towns or villages - only Mind Flayer hives beneath the ground and strange, non-Euclidian structures that extend above ground, including an impossibly tall mountain that seems to curve toward you if you look up at it.

Most 5e books only refer to stat blocks and the like within itself and the core three. Van Richten's breaks this rule occasionally, suggesting using existing stat blocks in Curse of Strahd for folks like the eponymous Rudolph van Richten (though it also provides an alternative in case you don't have CoS).

When it comes to Bluetspur, I'd highly recommend using some of the expanded Mind Flayer stats for monsters in Bluetspur, such as using the Ulitharid for the rival Mind Flayer seeking to undermine the God-Brain. (You could also use Dyrrn from Eberron if you want to get real nasty with it.)

The one possible exception is actually the Elder Brain stat block. The Elder Brain is CR 14, which might be fine if you're never going beyond tier 2, but Bluetspur seems like one domain where you could genuinely go all the way to tier 4 in Ravenloft. (Sidenote: I think I might have a campaign that goes 1-10 in Ravenloft and make 11-20 pure Planescape - maybe have a Relentless Killer antagonist who thinks they're a demon lord, until the actual, let's say Baphomet, shows up and destroys him for his arrogance, and then sucks the party into the Abyss.)

The God-Brain is no ordinary Elder Brain, and the chapter advises you to treat it more like a sentient place or object than a monster to fight - I'd use the regional effects from the Elder Brain and its telepathic link system, but make it essentially impossible to fight the damn thing.

However: here's the key with Bluetspur: The players don't have to go there for it to be scary as hell.

In the Lovecraft story The Shadow Out of Time, the narrator describes an experience in which he was witnessed to develop a completely different personality for a couple-year period, and he explains that what had actually happened was that an alien consciousness from the distant, distant past had actually swapped bodies with him, sending him back to a pre-human civilization he refers to as the Great Race of Yith, who have survived for countless eons by body-surfing into the future, with the implication that the extinction they experienced in the past might occur only after they've taken over humanity and left human minds to die off by their adversaries (despite this, the Yithians are one of the more benign creatures of Lovecraft's fiction.)

The Mind Flayers of Bluetspur are able to psionically project into other domains, abducting individuals from those realms and experimenting on them, possibly converting them via cereomorphosis into new Mind Flayers - they've experimented with introducing vampiric blood to mind flayers, creating Mind Flayer vampires who feed on others before going to the God-Brain's brine pool and getting dissolved within to nourish the ailing Elder Brain.

The tables in the Bluetspur section are 100% horrifying nightmares - one of the major themes is that the Mind Flayers constantly write over memories of abductions to their realm, so that an individual (including player characters) might have fragmentary, repressed memories that return to them (I think the one that blew me away the most was "you have a memory of a figure in front of you being deemed "acceptable," and was taken away, while you were deemed "unacceptable," and were left behind").

Adventure hooks for the domain include things like discovering a barn in which all the livestock has exploded, or a character discovers a metal device implanted under their skin. Full on alien abduction stuff here, which is a very different vibe than your typical D&D adventure, but makes 100% sense given what Mind Flayers are.

Just as another example of a piece of fiction that deals with these themes that stuck with me, the Star Trek: The Next Generation episode "Schisms," which is one of the creepiest episodes in that series. In it, Commander Riker along with some junior officers are having trouble sleeping, and eventually they discover that beings that live in subspace (the sort of alternate plane of existence that faster-than-light communication is broadcast through) have been taking crew members and experimenting on them, leaving subtle scars where, for instance, an arm was detached and then re-attached just slightly off its original position. And then, of course, there's basically all of the X-Files.

Bluetspur hits Cosmic, Body, and Psychological horror really well, and I think that it would make for a great "final adventure" for a campaign.