Void portal orchestrator, turned Targonians against one another, so balance would tip off
...and voidlings could take over Runeterra!
In Aurelion's lore we see how Targonians tricked him by giving him a splendorous crown of star-gems, which purpose was to control him.
They had a massive beast-weapon on a leash.
Now, there's also an obvious villain behind the scenes, to what extent have the events that played out, been manipulated by him?
"A pulsing sickness lingers in the air. From the epicenter of the corruption, I feel a will… hungry and indomitable, and far from the typical mindlessness I’m accustomed to from these Voidborn aberrations. The pulsating wound on reality yawns and buckles, distorting and warping all it touches. Whatever exists on the other side is laughing".
A villain seeing his plans roll out accordingly laughting maniacally?
It was implied I think, that the Targonians were in disagreeance: "(...) Then, he felt it–a weakening in his unwilling pact. The voices from the crown grew sporadic, clashing, arguing with each other while some fell ominously silent. An unknown catastrophe he could not fathom had thrown off the balance of those who bound him. They were scattered and distracted. Hope crept into his heart".
And with this last line,
"All will learn what fate befalls those who strive to steal for themselves the power of a cosmic dragon."
It makes me think that at the end of the story, he will be free, which means Targonians don't hold such power anymore.
Moving on,
"From what Diana could glean from the fragmentary texts, the Lunari spoke of the eternal cycle — night and day, sun and moon — as essential for universal harmony."
Essential for universal harmony. Essential. Without it, what? A crumbling of reality, a fissure ideal for voidling proliferation?
More of Diana's lore,
"Inside, the narrow cleft widened into the crumbling ruin of what might once have been a temple or vast audience chamber. Its crumbling walls were painted in faded frescoes depicting warriors of silver and gold fighting back to back against an unending host of grotesque monsters as the sky rained comets of searing light."
Solari and Lunari fighting off the Voidlings, in older times, when Aurelion was under their control. But I don't think he is anymore!
Maybe the Targonians inner conflicts (Solari vs Lunari) were part of someone's plot to throw off the balance and set loose Aurelion, leaving the planet vulnerable... to an upcoming void invasion.
Poorly written I know sorry but I hope you get the picture which is, someone orchestrated the separation of Solari and Lunari.