I'm preparing to run Curse of Strahd, and one of my players wants to play as a Shadar-kai who is loyal to the Raven Queen.
In the adventure's set up, it says (p. 9):
Unwilling to go the way of his father, Strahd studied magic and forged a pact with the Dark Powers of the Shadowfell in return for the promise of immortality.
For obvious reasons, I'd like to know more about the connection between these Dark Powers of the Shadowfell and Strahd.
I've looked at the related question What are the "Dark Powers"? but it seems to be an old question based on old D&D lore (from 2e and 3e) that doesn't help in this regard, because the Shadowfell appears to have been introduced in 4e (and remains in 5e). Hence this question about the 5e adventure Curse of Strahd and the 4e/5e Shadowfell is not a duplicate of that question.
Rather than focusing on the Dark Powers (since that would make this a duplicate), I want to instead better understand the relationship between Strahd, Barovia and the Shadowfell, especially if the Raven Queen is supposed to be in any way related to any of this. If it turns out that she is related to this, then this player's backstory will have to include why the Raven Queen is sending their PC to ruin Strahd's day (especially since, AFAIK, the Raven Queen dislikes sentient undead), whereas otherwise the "Raven Queen dislikes sentient undead" thing is probably enough on its own.
So, in what way is the Shadowfell related to Strahd, and the plane of Barovia (if at all)?
I'm chiefly interested in any 5e lore that might help me understand the connections between the Shadowfell and Barovia/Strahd, but 4e lore is acceptable as well. 3e and older lore I won't shun if it can help in any way, but the fact that the Shadowfell didn't exist back then implies that answers must contain at least some 4e/5e lore as well for such answers to be helpful.
Best Answer
The Domains of Dread are in the Shadowfell
The plane of Barovia is a Domain of Dread, and Domains of Dread are located physically in the Shadowfell, in a far remote corner of the plane and hidden away from the rest of the Shadowfell. (DMG 51–52)
The Raven Queen is also located in the Shadowfell, where she controls several domains, but that is where the similarities end. She is not related to Strahd in any way and isn't mentioned in the campaign at all.
If the Shadowfell is planet Earth, then the Raven Queen owns some land in Russia and Strahd lives in Brazil and you are looking for a connection between the two. The connection doesn't really go much deeper than "they're on the same plane". The Raven Queen has no power over Barovia and most likely doesn't even know much more than "it exists", because information and people do not leave the Domains of Dread very often.
Residents of the Shadowfell have no easier a time entering or leaving Barovia than normal residents of the material plan do. In fact, there are multiple Domains of Dread, where others are trapped just like Strahd is, and even those Domains of Dread will have little to no interaction with one-another. They are essentially mini-planes with hardly any way of entering or leaving.