The vibe I got from Mordenkainen's Sorrowsworn was that of a Silent Hill homage. The monsters, both in lore and art style, seemed very much like the denizens of Silent Hill.
I'm not quite sure I'd lump the Raven Queen in with that.
In my last PoL campaign I created a couple 'Shadowlords', which were the Shadowfell's equivalent to the Archfey, and one of them (Mourne, from the 4E 'Shades of Darkness' article in Dragon Mag) became the Warlock patron of one of my PCs that had the Dark pact.
In my upcoming PoL campaign, I'm adding a couple others. Acererak and Vecna will both be Shadowlords as well.
Aren't Shadowlords the trapped rulers of Ravenloft, or is that Dark Lords?
Darklords (one word) are the trapped domain rulers in Ravenloft. The Dark Powers (two words) are the unknown entities/forces behind the existence of the Domain of Dread itself. I don't think "Shadowlords" has any existing etymology in the game, though the idea of Shadowfell-equivalents of Archfey is a cool one.
As is, come to think of it, the idea of some of the more powerful Ravenloft Darklords serving as warlock patrons.