According to Chronicle, the Void Lords - beings of unfathomable power and nature - sent the Old Gods with one purpose: find a Titan World-Soul and corrupt it to create a Void Titan that could act as their agent in rendering the entire universe Void.
When Sargeras discovered an ensouled world thoroughly corrupted by its own Old Gods, he didn't hesitate in destroying the planet, preventing this Void Titan from being created. This event, and the Pantheon's reaction to it, eventually led Sargeras to create the Burning Legion, freeing the demons he had spent eons fighting and imprisoning.
Sargeras decided to do to the universe what he had done with the planet - to burn everything away with Fel Fire.
But there are some problems here. First off, the Legion ostensibly is all about preventing the Void from corrupting the universe, but they freely use void magic almost as much as they use Fel magic. Second, the Void Lords want to unmake the universe, being "avatars of non-existence," according to Star Augur Etraeus. Is destroying the universe not doing their work for them? (That's a legitimate question, not a rhetorical one, because I don't know if "ashen embers still faintly glowing green" fits the bill of "existence" that the Void wants to stop having.)
With 7.3 going up on the PTR and some big details emerging, I think we are starting to get more evidence that it's all a bit more complicated.
Spoilers to follow.
The final boss of what I assume is the final raid of the expansion is "Argus the Unmaker." Given that this is the planet Argus, a character with the same name would seem heavily implied to be a Titan. It is not difficult to imagine that Argus, like Azeroth, has a World Soul within it, but Argus has been under demonic occupation for the last 25,000 years.
The image of Argus the Unmaker actually looks fairly like a healthy Titan (compared with the other models of the pantheon who are also up.) He certainly does not seem to have flame all around him like illustrations of Sargeras have.
However, a lot of models found in the 7.3 PTR seem to involve void corruption and tentacle-like structures that seem to be made of dark blue space and stars. Could Argus be the sought-after Void Titan that the Void Lords have been trying to create?
And if so, how the hell was he able to grow and develop on a planet that has served as headquarters for the Burning Legion, whose primary purpose (supposedly) is the prevention of exactly that?
One theory I've seen is that Sargeras' body was destroyed when the Well of Eternity collapsed on him in the War of the Ancients. He has lived on in spirit form, but perhaps the reason (or at least one of them) that he came to Argus was so that he could hijack the embryonic Titan's body.
Argus and its Eredar had a profoundly advanced civilization, its people being masters of science, magic, art, and all sorts of things. It doesn't seem too hard to imagine that the Titans might have shaped their world long ago, perhaps because of the existence of this world-soul. That would also explain why Argus was in Sargeras' rolodex when he went to shore up the leadership of the Legion.
But why is there this void corruption here?
I have a couple theories.
One is that Sargeras himself has been slowly corrupted by the void. Perhaps if he installed himself in Argus, he left himself vulnerable to perhaps not Old Gods but some other form of void corruption.
More likely (or working in tandem with the last idea) is that perhaps some of the demons Sargeras recruited have not been entirely loyal. We know the Nathrezim had been working for the Old Gods when he first found them. Perhaps they have been all along (Varimathras is a boss in the raid.)
Anyway, I'm eagerly, eagerly awaiting more info about this, and I think the upsurge of void stuff is really pointing pretty clearly to an Old God expansion to follow Legion.