I think her end goal is to absorb N’zoth into the Black Blade and use his power to gain complete dominion over the Shadowlands. The line she says when she raises Derek Proudmoore, “death is mine to master”, is what makes me think this. I mean, compare that to things she said in the past like “what joy is there in this curse.” She’s come to terms with her undeath and has been practicing being the lady of the dead throughout all of BFA. BFA ends with us fighting whoever is in charge of the shadowlands right now (Bwonsamdi’s boss) to clear the way for the lesser evil, Sylvanas, to take his/her place.

N’zoth knows he can’t mind control her, so she is a legitimate threat to do something like this to him if Azshara isn’t able to protect his prison. Ironically.

Xal’atath is on board with this plan because it freed her from the blade. We will have to deal with her later.

This whole thing might have been Yogg-Saron’s idea, since Sylvanas jumped to her death and was impaled on Saronite. Whispers and void magic doesn’t work, but a saronite spike through the heart is a different thing. Retconning that as being the only way an Old God can influence the undead isn’t objectionable to me.