While the Warcraft universe is multifaceted - a universe with myriad powers that might be opposed or cooperative with one another - if there is any Big Bad to the whole franchise, it's Sargeras.
Sargeras fits the Western Satanic archetype - he was once the paragon of a benevolent group of beings. While Aman'thul was the leader of their order, he wasn't a fundamentally different being than the rest of the Pantheon - more akin to a Zeus or Odin-like figure than the Abrahamic God.
Still, Sargeras fits into the analogy of the Devil in that he was once a great force of good that then turned to evil.
What is interesting here is that Sargeras considered his actions necessary - at least at first, he believed that he was serving the greater good.
Because in some fundamental ways, Sargeras does not fit this Satanic archetype. He is not the source of evil. That is really more accurately the mysterious collective Void Lords.
We know very little about the Void Lords. They are beings of pure shadow, and given they are the creators of the Old Gods, we can only assume that they are utterly alien. Indeed, while we're using a plural to refer to them, for all we know their nature might transcend the difference between singular and plural states.
It is clear that they have influence upon the universe (hence the Old Gods,) but if we take the description of their home - the Void - literally, they seem to exist outside of comprehensible logic. A Void is by definition empty, and so to say that the Void Lords exist within the Void is to say that, in a sense, they don't really exist.
More precisely, though, we might say that they are the embodiment of non-existence.
While this actually means they play a very similar role to stuff within my own fiction, I'll hold off on referring to that as, well, it's significantly more obscure than other things I could reference (and it's also weird to refer to one's own inventions in this way.) Instead, I think a good analogue would be figures within the podcast Welcome to Night Vale.
If you haven't listened to it and you like dark humor, Lovecraftian horror, and tin-foil-hat conspiracy theories, I highly recommend it.
Anyway, they had a plot recently in which the town is under attack by "Strangers" who serve an evil force taking an unexpected form. The evil force wants nothing - it's not that it doesn't want anything. It's that it literally wants nothing. There are things everywhere, and it would rather that those things were no longer there.
I suspect that the Void Lords in Warcraft are looking for something similar. The Old Gods and their corruption are a means to an end. Eventually, they want the universe to be a lifeless void like their own home.
Sargeras wants to burn the universe - in theory to deprive the Void Lords and the Old Gods from having access to any Titan world souls that they could corrupt.
But is Sargeras actually doing exactly what the Void Lords want? By destroying the universe, he is creating the absence of life that the Void Lords presumably want to create.
We've seen that the Old Gods don't really have any problem helping the Legion. They created the Emerald Nightmare, and when Xavius - a demon - came in, rather than destroying them for threatening their project, they instead seem to have handed him the reins.
The Legion wants to destroy everything to keep it from falling into the corruption of the Old Gods, but the Old Gods don't actually have self-preservation as a goal or desire. When their job is done, whatever flesh or sentience they have would presumably be lost in the endless void along with everything else.
So that's one possibility - that perhaps the Old Gods are actually rooting for Sargeras, even though Sargeras holds them in contempt.
But there's also another possibility:
On the Chronicle diagram of the primal forces of the Warcraft universe, it was somewhat surprising to see that Light and Chaos were right next to each other, while Order and Void were neighbors.
Yet there is, in a sense, a kind of logic to that.
For one thing, if you go another step down, you can see that the Element of Fire is roughly equidistant from Holy and Fel magics. And that's reflected in game. Priests have Holy Fire, and Ret Paladins use the Ashbringer to disintegrate demons and the undead. The Light burns the undead. Light itself is, of course, a form of energy (nitpicking physicists might argue that's not strictly true, but you get what I mean.) Likewise, Warlocks and Demon Hunters do a lot of burning with Fel fire.
And in fact, you could argue that while they're usually in opposition to one another, demons and paladins are both zealots - they channel a passionate fury to use agains their enemies. Emotion plays a huge role in both the power of the Light and also the wrath of the Fel (consider that Demon Hunters use Fury and Pain as their resources.)
On the opposite side, however, you can also draw connections between Order and the Void. Arcane magic is practiced with care - Mages are scholars, deriving their power not from faith or passion but from study and intellect. While a Paladin will fight at her best when she sets aside all doubts and fears and submits to her faith in the Light, a Mage must remain calm and think logically, presumably performing all sorts of complex calculations and adjustments as he casts each spell.
I would imagine that those who employ the power of the Void, such as Shadow Priests, have a similar mindset. Shadow Priests assault their opponents' minds with horror and madness. Now, their resource is called Insanity, but is it the Shadow Priest who actually goes insane? Consider that the farther into Void Form they get, the harder it is to hold on to that "Insanity."
Instead, perhaps we can imagine that a Shadow Priest must confront the paradox of the Void - that there exists this emptiness that nevertheless has something within it - something that exists only in that it does not exist, yet clearly does exist enough to influence the universe. I suspect that "Insanity" is really the Shadow Priest's ability to think in such abstractions that they are able to conceive this paradox, and eventually channel this paradox with their mind. But as it goes on, their hold on the paradox begins to slip, which accounts for why they cannot stay in Void Form forever.
The point being - this is not an exercise in Faith and Passion, but rather in the ability to forcibly set aside instinctive intellectual tools like, for example, the emotion of fear, and instead remain coldly logical, thinking entirely within extreme abstraction.
The Titans struggled to deal with the Old Gods because their own "orderly" universe was, in fact, too similar to the lifeless one that the Void Lords desired. Sargeras sought out a means to counter the Void.
The tragedy is that he only barely missed the mark.
In a better (though far less entertaining for those of us on the outside) universe, Sargeras would have turned to the Holy Light in order to fend off the Old Gods. He would have infused the threatened World Souls with such Holy radiance that it would have burned these eldritch behemoths away. The Light burns, but in doing so, it purifies, leaving behind the good and healthy while the corruption is eliminated.
Instead, Sargeras turned to the Fel.
The Fel is almost like radiation (again, physicists, I know that light technically is radiation, but you get what I mean) - it burns things away, yes, but it leaves harmful energy in everything it touches. It is an imprecise tool, and its unpredictable and chaotic destruction winds up becoming its own kind of corruption.
A triumphant Sargeras might, in fact, create a universe that is the opposite of the will of the Void Lords - they seek a cold and lifeless universe. He will give them an endless raging inferno.