Let them come.
At Icathia, the Sunborn Ascended and their human followers were the sole defenders of Runeterra from the Voidborn. The magical mental deterioration, along with the regular mental deterioration expected from prolonged fighting, was also solely theirs to bear.
The Void is returning. But now there are many incredibly strong mortal and immortal allies to assist Azir, Nasus, and I sure as hell hope reascended forms of the remaining Darkin. (Since the return of the Sun Disc means the return of the ascension magic that has been gone for millenia, and the return of Azir may just be enough to unbreak the minds of his remaining brethren)
The Freljordian demigods are currently in preparation for the return of The Watchers (who if you weren't aware aren't just a Freljordian threat, but actually control the Voidborn).
At least one Vastayan we know of, and probably the others powerful enough to be "champions", is/are on par with intelligent Voidborn.
Demons would need to protect the planet, as they need humans to thrive and survive, or are being used.
Humans have proven that they can overcome the magic of the Void.
Wields power capable of devastating entire nations, and the whole reason he's collecting it is to protect the planet.
Is the true hero who wields the legendary hammer of Orlon.
An enormous behemoth capable of nullifying magic, which seems to be the main form of attack for the Voidborn.
Highly advanced Hex/Chem technology and modern munitions.
The Aspects of Targon who for some reason didn't help out the first time.
Who is here specifically to protect the fabric of the universe which is being threatened by the crossing over of the Void.
And hell, if none of that works we got this motherfucker to turn the mortals into wraiths, and keep on fighting.
Let the Void return. Runeterra is ready.
(I'm not saying we know they'll win-- despite the fact we do know for a fact they have to unless it's used as the excuse to shut off the servers--, just that there's a lot of defense this time around that didn't exist before)
catches a glimpse/senses the power of the being(s) that are in control in the void, and he's scared. the literal creator of stars who is Targon's attack dragon that closes rifts for them, is scared of what's in the void. Runeterra is gonna need some pretty heavy hitters to even contend, and I feel like the factions are too divided or absorbed in their own stuff to be able to form a cohesive and effective alliance against the void. at least not right away. If anything it will be like WWII, and the tides won't turn until one of the major powers decides that fighting for the whole world is more important than what they have going on. (not exactly like WWII, unless said power just doesn't help until the void attacks them directly. loose similarity I guess)