I’m gonna add a potential plot twist to this.
We know the Black Empire was something that Odyn went to great lengths to paint as something irreversibly terrible, and certainly it was, but we’re told in previous Lore that the Void cannot breach our reality.
So where did the Old Gods come from, if the Void is so 
insane that even the other Primal Powers have sealed it off from our layer of reality?
But we already know that the Chronicles are all half-truths and lies-by-omission, and the Pilgrims in Zerith Mortis are quick to point out our Planar Charts we recovered from the Discs of Norgannon are horribly incorrect.
Putting all this together, and I’m in a staggering amount of pain and the pain-killers aren’t really working right now, but hear me out.
What if the Black Empire is the remains of a Fallen Naaru Empire?
We know Naaru who come into extended contact with Mortals fall to the Void-part of their life-cycle quicker than those who keep their distance and interact only via visions and proxies. We now know there’s a mysterious crystal full of Light that the Cave-Arathai are using as a replacement sun, and is responsible for the bountiful life in their gigantic cavern. We know Naaru can survive the removal of their Core, as evidenced by Xe’ra.
We know Sargeras was aiming for something.
We know that the Primal Powers can all be used to produce the same effects, its just that some find it easier to do some things than others. Cauliflower herself is proof that Undeath is possible through the Light, and we’ve never seen what happens when she uses the Light in her new state of being.
What if, and I’m aware this is out there, but what if there was another Prime Naaru like Xe’ra on Azeroth, and it was falling to its Void State quick. So it ejects its Core and infuses it with Azerite in an attempt to avoid becoming a Void Lord of incredible power, but the rest of the Naaru left behind is now uncontrolled and flooded with Void energy. Mortals, infested with the madness of the Void, channel more Void and Souls into the fragments of the Naaru’s crystaline form, and new beings arise, cut off from their Core and flooded with the will of the Void Lords.
It would explain why the Old Gods despise one another, seek to consume each other where possible, and each have unique traits that would work best when used in tandem, rather than against each other.
C’Thun is the Old God of Madness and Chaos, and was reputed to have the greatest grasp of Future-sight amongst their kin, but this also left him incredibly vulnerable because he could get lost in his own sauce and be unable to advise his servants on the correct course of action in time. and the Light offers prophetic visions to its followers, even if those visions are not strictly true, but rather what the Light wishes its followers to do …
Yogg’Saron is the Old God of Infection and Corruption, and not only created the Curse of Flesh, but actively spread his blood throughout the lands that would become Northrend because he knew that so long as a single scrap of his remains existed, he could never be truly banished. His claims to be the ‘God of Death’ might actually be linked to his Black Blood (fresh, supposedly unusable Old God blood that defied all attempts by all forms of magic and artifice to be shaped by the Argent Crusade) being etched with the Runes of Domination and being used to build the indestructible fortress of Icecrown Citadel over the engines that the Jailer planned to funnel all of the World Soul’s life-blood out to fuel his terrible plan for all of Reality. Light causes life to flourish, unity to grow and the better emotions of the mortals to become stronger over time …
Y’shaarj was the strongest of the Old Gods, said to possess seven heads and be the master of emotions, able to draw out the darkest impulses of even the most noble and stoic of souls and turn staunch allies against each other in the blink of an eye. That strength would come back to bite him as his ability to hold off the Titan-Keepers and -Watchers provoked Aman’thul himself to reach down and tear Y’shaarj free, and it would not be the first time Aman’thul would tear things not of Order free from Azeroth, heedless of the cost to the World Soul so long as Order controlled the planet and could nurture it into a new Titan for his Pantheon. And the Light answers the strongest of wills, and the most potent of emotions …
Finally we get to the weakest of the Old Gods, N’Zothis the Old God of Manipulation and Deceit, known as the Dweller in the Deep, the Deep One and the Knowing Presence. Every action N’Zoth took was calculated and exploited for maximum effect, even going so far to lose to Y’shaarj at the Broken Shore in the ancient past on purpose, and has actively guided, shaped and traded dark and forbidden knowledge to its followers without actively driving them insane or turning them into horrible flesh-monsters because it knows covert servants are far more useful than an army of unstable, gibbering lunatics. Even N’Zoth’s version of the Black Empire was strangely ordered and controlled compared to the armies of C’Thun and Yogg’Saron, who relied upon overwhelming numbers and power rather than the skills and knowledge of those servants. And the Light offers clarity, surety of purpose and wisdom to those who are one with it …
This is just a random, off-the-cuff thought, but it would make a lot of sense that that’s why the Titans moved so damn quick when they found the Black Empire. Light is Order, even if it is not the Arcane that they favor, and if Hearthstone is Canon, we already have a Light Titan. We’ve also seen Tyr wield the Light freely, and a Titan Watcher in Uldum was specifically crafted to use the Light to reseed organic life across the world in the event of a Re-Origination … and considering we now know that Aman’thul saw Life as inherently Chaotic and unworthy of the Titans’ protection, that now makes sense as to why they would use Light, rather than Life, to restore organic life to the planet. Light is Order, and can be counted upon to stick to structure and stability rather than just Yolo’ing it.
Edit: Sorry
Adding further to this, the Void we encounter in Bastion is almost entirely energy-based from memory, with only the Vile Oozelings being organic, but they might be native Bastion life-forms mutated by Void into jellified flesh rather than actively native Void entities like the other ‘Pure Void’ creatures.
Void, outside of our plane of existence, seems to be purely energy-based. The Void entities we find on Azeroth seem to be residual matter reshaped and possessed by Void entities from the original biological mass of the Old Gods, and if the Void is purely energy based, then that could mean they evolved from crystaline matter, which would also explain their near-immortality and tremendous physical endurance. Literally, Silicone-based life-forms rizz all over us Carbon-based ones, and whatever the Naaru are made out of, it is not only absurdly conductive for energies, but can store tremendous amounts of those energies, and can even absorb the energies of other Primal Powers, like the Naaru we meet in Revendreth.
Adding to this yet again, Z’rali was permanently affected by ‘dark magic’ and twisted by Sire Denathrius and his Dread Lords, and is considered an abomination by the other Naaru. If this holds true, the fragments of a Elder or Prime Naaru would likewise be considered abominations by the rest of the Naaru, hence why the Core remained buried, and the other Naaru refused to come near Azeroth until they literally crash-landed with the Draenei, and that one single Naaru stated in the Exodar and refused to move the entirety of its time on Azeroth until killed by Rakeesh.
It makes me wonder if the Light gave Velen visions of putting Mu’ru’s extinguished Core into the Sunwell because it would create an influx of Light into the Azerite of Azeroth, and could potentially both funnel more Light to the Azerite-infused Core of the Elder/Prime Naaru and be used to create more Light-worshippers on Azeroth to re-create that Empire of Light without putting actual living Naaru at risk of falling to their Void states.