Let me start with a spoiler alert, I will discuss lore from the PTR.
So looking at the story surrounding N’zoth and Xal’atath in 8.1.5, we as the player is given the “gift of N’zoth”, and it seems like he consider us as potential servants.
However, death and the void are being described as enemys, and it is clear that N’zoth is fighting against the champions of death, like Sylvanas.
But why would N’zoth then work with forsaken players? Aren’t they supose to be his enemies by default?
The Old Gods are fighting against everyone, and there have been undead cultists since Cataclysm anyway. One of the Twilight high-ranking members was an undead called Domina.
No, they aren’t necessarily. That’s a common interpretation of the void whispers telling Alleria to kill Sylvanas, because she is the “true enemy”, but it is by no means confirmed.
Well the old gods can probably not use the souls of undead creatures the same manner as ordinary souls. The state of undeath are afterall a soul which are “dislocated” from the ordinary circle of life and thus already corrupted. This do not mean that a undead are useless to them, especially the forsaken which have “free will” at least to a fairly high degree.
Ofcourse freshly risen undead will always come with a manner of “loyalty” to their reanimator. Which means we don’t really know how many of the forsaken affiliated NPC’s that don’t truly have “free will” or are feeling some unnatural loyalty to Sylvanas due to the reanimation process. But the player character at least do have free will for the intents and purposes of the story.
N’zoth are a old god who truly enjoy to spread madness and really disslike things like honor, morality and restraint. It’s part of the mythos that N’zoth won’t rise before “the world has been dyed his color”.
Which means N’zoth desires a world were people are not bound to things like morality, loyalty or restraint. N’zoth keep on whispering about “all eyes shall be open”, this relates to those souls which do not live a life following the common conception of morality, shows restraint or consideration of others etc.
That’s why the story of BfA focus alot on morality, lost hope and well carnage in general. N’zoth wants to free people from the chains which morality, restraint and justice etc. place upon them and replace these values with things he really enjoys. According to the mythos N’zoth(Cthulhu) won’t rise before; the mortals are beings beyond good and evil, with laws and morals thrown aside while the kill eachother in joy.
Undead beings however… most of them are tied to their master/necromancer and thus they are already in a state which entails these values. But not under N’zoths “influence” which makes these sort of beings both the necromancer and it’s spawns his enemies.
The souls are dislocated by shadow magic which prevents their souls from both leaving and properly attach itself to the body. The shadow magic are the “dislocating” element which can be used to; corrupt the soul, making a undead a mindless slave. Aswell as it keeps the soul in a state beyond entropy, which means that the shadowlands; Void, Light or beings which harbors souls can’t claim them.
It’s hard to know what truly happends to undeads when the magical link between their manifested form and soul are broken. I think most of them just return to the shadowlands were they will see their past life like ordinary mortals do.
In the case of Arthas and Sylvanas, it may simply have been a “damned” place due to how they have lived their lives. I doubt the “light” will take them in, however the “void” might be interested in their souls. Given all of Sylvanas visions maybe the void had plans for her, it’s hard to know since not many forsaken speak of the moment of their 2nd death since well… their dead.
Given that most undeads have done loathesome things either during the stage of their reanimation; being mindless rage driven cannibals or while in service of the lich king; done many evil or immoral deeds. The afterlife might not be a peacefull experience for these souls. But not all forsaken are “damned”.