The void caverns are giant voidborn
First off, this is a theory. I'm not claiming that what I'm about to say is fact. That being said, what evidence do we have to support that theory?
The most obvious argument is that they move, and not just in any random direction, but directly towards anything consumable. This can be seen in "Where Icathia Once Stood" and "Breach", although it is never explicitly stated. The caverns don't just attack things randomly as well, but focus on the biggest target in reach before moving on to the next one. In "Where Icathia Once Stood" the they first devour the Shuriman army, which has the largest amount of people and magic. After the Shuriman army is gone it shifts it's focus to Icathia, which is the next largest collection of people. However, this prioritization could potentially just be for the sake of the story.
Secondly, they need to feed, which has also been known for some time. The exact quote from the universe page is:
One of the few known ways to combat the Void is to starve it. With no organic or magical sustenance nearby, the Void’s material growth will slow, until it eventually falls into a dormant state.
This tells us that if the caverns have "food", they grow, which is a behavior exhibited by other voidlings like Cho'Gath. Similarly, a lack of food doesn't kill the caverns and other voidborn but leaves them in a dormant state (as confirmed by the void universe page). Using "Where Icathia once stood" as an example again, we can see the void caverns being fed magic by Icathian priests. (Theory, not fact) The effects of the void are present long before it actually bursts from the ground, which might mean that it is already present further underground. The magic fed to the caverns allows them to grow and reach for the surface. The "food" provided by the Shuriman army (including Ascended) and Icathia (the city) allows the caverns to stretch far along the land, leading to the huge area of influence it currently has in Icathia (the region).
And lastly, every voidborn has a so-called "void heart". They are usually the weak point of voidborn as seen in "Breach". That's nothing new. However, we recently learned through Kassadin's story "Whom Does the Desert Know?" that the caverns are no different.
It was Horok who first dared to face you down here, in the darkness, away from the light of the sun that had given him his strength. It was Horok who first bore the Nether Blade unto the Void’s vile heart.
And it was Horok who showed his brothers and sisters how to defeat the abyss.
[...]
This is my land. Such horrors walk here now, openly, and I cannot allow it. I will plunge this blade into the creeping nothingness beneath Shurima, as I have a dozen times before.
At first I thought this to be metaphorical until Scathlocke dropped this comment:
Kassadin seems to be one of the few people in the world right now who even knows that Void-hearts exist, and they can be destroyed.
The caverns live. They are vulnerable.
Not only does that explain how the Ascended managed to drive the void back before Shurima's fall, but it also gives us another option to defeat the void other than compromising on the dormant state after you starve it. Too bad Kassadin is currently on a "nothing left to lose" void killing spree and doesn't seem to care about sharing his knowledge.
But if he were to meet up with Kai'Sa by chance...
Relevant sources: Universe Void Region Image Gallery Where Icathia Once Stood Whom Does the Desert Know? Breach The Girl Who Came Back Scathlocke's comment
