So maybe I missed some lore, and I have not run Curse of Strahd, but why wouldn't Azalin be the lord of Darkon anymore? Always has been one of more favorite characters.
So here is my theory (and I'll show my work best I can).
In 2e, there was an event called the Requiem, in which Azalin tried to escape Ravenloft by becoming a Dark Power. It didn't go as planned, and he scattered his essence throughout Darkon until he could gather enough of himself together to reform. In the meantime, demi-lords (six of them) controlled the six regions of Darkon and the capital of Il Aluk was turned into Necropolis, with Death as its Darklord (Death being a reaper-like entity made from a failed Azalin clone). Darkon during that era was divided and leaderless and on the verge of being reclaimed by the dead.
Flash forward. Black Dice Society has stated Darkon experienced the Hour of Ascension, where the dead are reclaiming the land. Azalin Rex is gone, and factions fight for control of the land. Sound familiar? Thought so. However, this doesn't appear to be the Requiem as Il Aluk still stands, there is no Death, and Azalin has appeared in the stream, stating he has slipped his bonds and goes and visits his old Friend Strahd. Further, we have a nice piece of art in the art thread of Strahd and Azalin having a civil conversation about the meaning of life. Further, Azalin is name checked both as a Patron in Tasha and in the Undead Warlock UA.
THEORY
Darkon's Hour of Ascension is basically replicating the post-Requiem Darkon, but without the Necropolis elements. Azalin is around, but he's not the "Lord of Darkon" anymore as he's found a way to wander around the Domains (alternatively, he's locked out of his own domain and must watch it spiral into civil war, a sad curse for a king). So, he strikes deals with PCs to move chess pieces on the board. So Azalin will be in there, but in a way he can stir more trouble beyond his domain...
EDIT: So see the name "Firan Zal'honan" in the Mistwalker's section? Google it. I think that's settled.
DOUBLE EDIT: Also this.
https://pbs.twimg.com/media/E0VOLhMVgAQb_vj?format=jpg&name=large