Guess what? I have more questions.
Darkling Coverns. Apparently they're a secret, but they have a army list and fight alongside Cities of Sigmar. How does that work? How hidden are they really? Do they just turn up out of nowhere to help in fights? Or is their AL list more about putting old WHF models somewhere
Spoiler for possible future quest;
I have an idea that Sigmar knows about the DLCs, but tolerates them. If the players choose a Darkling Sorcerer as their character, Sigmar straight up revals he knows about the Covens, and forces they into agreeing to do a task for him
If you want a good worldbuilding resource for stuff like that, the core Soulbound RPG book is a good one. It gives a brief overview on many of the Free City organisations.
In terms of the Darkling Covens, it should be noted that the Darkling Sorceresses are the highest level, and they are not the only ones in this group. There are hordes of soldiers and thralls that fall under the Covenite Sorceresses who serve as the hencheman and servants of the group when they need something done, and they have lots of spies, informants and double agents and use a mixture of bribery, blackmail and mind controlling magic to assert their will. Mind control is a pretty essential part of their kit I should say, and I should also note that Ulgu explicitly has mind control powers. The Darkling Covens consists of Executioners, Darkshards, Dreadspears, Bleakswords and Black Guard alongside the Sorceresses.
This is probably partly because the Lore of Dark Magic is no longer used by the Aelves. Dark Aelves (or their equivalent) use the Lore of Shadow, Ulgu, while the Lumineth Realm Lords use the Lore of Hysh instead of High Magic, with some additional associated lores.
If you want to reconcile the secret aspect with the fact that they participate in fights, you can easily justify it with a mix of thralls, mind magic, intrigue, background skullduggery, and a number of different methods. If a Sorceress pops up in the field, she may be notable, but illusion is their specialty.
The reason the Darkling Covens are largely secret is because they never really integrated with Azyrite culture when they migrated over to Azyrheim back during the Age of Chaos, so they sought to seed their influence in the background and avoid the Order of Azyr, who are the Witch Hunter equivalents of the Free Cities.
