The Void is really the wellspring of all problems in the Warcraft universe, but it's also necessary for there to be existence in the first place. Even with six primal forces at work, it seems there was originally just Light, and when that receded from certain parts of the cosmos, it left Void (and the clashing of Light and Void then created the other aspects of the universe, like the Twisting Nether and the Great Dark Beyond.)
Star Augur Etraeus takes us on a tour of the cosmos in his fight - first to the mundane but cold Great Dark Beyond, then the chaotic Twisting Nether, and finally into the horrific Void. Etraeus describes the "true horror of our reality" as "avatars of non-existence, knowing only hunger."
That seems to confirm the notion that the Void is truly a kind of paradox - a place defined by its not being a place, with beings in it defined by not existing.
There's a kind of irony here, which is that the Void has seen two major representatives in-game. There are the disgusting, fleshy, goopy Old Gods and their related corruption, and the sterile and cold Voidwalkers.
Voidwalkers are often classified as demons, especially as they are used by Warlocks. But Voidwalkers (and related Void beings) are fundamentally different from Imps, Felguards, and the like. While their origins seem to range from humanoids to beasts to elementals that were corrupted with Fel magic, the thing that seems to tie demons together is that their souls are bound to the Twisting Nether.
Almost all demons found themselves conscripted into the Burning Legion, which is a handy way of keeping track of them, and to be sure, Voidwalkers are employed by the Legion. Even though its purpose is theoretically to defeat the Void (by destroying any universe that the Void could corrupt,) the Legion is perfectly willing to employ Shadow Magic. We can't really call this out as hypocrisy though given that the heroes of Azeroth count Demon Hunters, Warlocks, Death Knights, and Shadow Priests among themselves. Indeed, the Legion's use of Shadow magic could easily be explained the same way we explain using it - it's powerful can can be turned to one's own purposes.
But that still means the Voidwalkers aren't exactly demons.
Or perhaps they are.
Here's the thing: in Warlords of Draenor, we saw Nerzhul-B using Shadow magic derived from K'ara, the injured and corrupted Naaru that hovered above Shadowmoon Valley (I wonder if this was how the valley and its clan got its name.) In addition to using necromancy (which is theoretically its own separate branch of magic but also seems to be accessible via Fel or Shadow magic,) Ner'zhul has Void beings that work for him.
The odd thing is that these void beings are classified as Aberrations, rather than demons.
But that makes sense - generally in WoW, Aberration (a creature type introduced in Mists of Pandaria's 5.2) refer either to oozes or to things associated with the Old Gods (including the Old Gods themselves.)
But if we are to assume that most if not all demons were originally some other kind of being, and that those types can extend from humanoids (e.g. Felguards) to beasts (e.g. Dreadstalkers) to elementals (e.g. Infernals) then perhaps it isn't impossible to imagine that an Aberration could also be converted. If all it takes to be classified as a demon is that their soul is bound to the Twisting Nether, perhaps that's what the demon-type Voidwalkers are.
There does seem to be an opposite number to the standard Voidwalker, which is the Lightspawn. Priests actually get to recruit a champion named Sol who is an entity of pure light.
But we've also seen a connection between more powerful void beings and Naaru. Namely, that when Naaru go dark, they seem to become Void Gods. We saw this happen with M'uru's transformation into Entropius, as well as with K'ure-B in Oshugun-B.
Now is this what the Void Lords are? It seems unlikely, given that the Void Lords created the Old Gods, who seem way more powerful than a single Naaru or anti-Naaru.
But there's also an interesting question here, which is why the Old Gods seem so very different from the Void beings we've encountered. The Old Gods are heavily associated with madness - both eliciting it and embodying it. The Void beings are horrific, and certainly we've seen things like the Pale arise in their presence, but the void beings themselves seem far less chaotic in nature.
And then we have to take a look at the fleshiness. The Void is, as we said earlier, a realm of nothingness. But the Old Gods and their associated eldritch monsters are decidedly fleshy. They're disgusting in a way that only organic material could ever be. And not only that, but they programmed in the Curse of Flesh to the formerly metal-and-stone-based Titan creations (though there seem to be more hints coming that the Curse of Flesh is not entirely original to the Old Gods.)
The Void Lords created the Old Gods to infect World-Souls because they couldn't do so directly. But given that the Void Lords are, as Etraeus said, "avatars of non-existence" and inhabit a realm of nothingness, how could they create the Old Gods in the first place?
Now, sure, you could simply say that the lack of explanation there just lends more to the creepy, Lovecraftian vibe of them. But perhaps there's a different explanation: Perhaps the Old Gods are something that existed, in some form, before being used by the Void.
What they were prior to becoming what they are is something I could only guess at - they could be biomass corrupted by the Void not entirely unlike the way that demons are corrupted by the Fel, or they could be things that were once Naaru-like beings. Or perhaps they were just some really odd alien life out there in the Great Dark Beyond.
It just seems odd that the Void - a primordial force that is about purity just as much as the Light - would manifest as beings that are the embodiment of impurity and, well, messiness.