I'm honestly hoping it turns out that there's more to the Void than 'no, they're totally not Tyranids from another plane of existence'.
The Will that Sol felt was obviously intelligent. So are the Voidborn. Intelligent things don't just go around destroying stuff for kicks and giggles. They have reason for what they do. Something motivates them. So here's a completely random concept based in part on Cho's selection quote.
"Death will not be the end for you. I have seen to it. For eternity, you are mine."
Imagine for a moment that the Void is a place wholly alien to the structured universe of Runeterra. We already know that Voidlings don't react quite right to physical forces they don't really 'get'. Sol told us as much. So let's take that a step further. In the Void, physical law does not exist in any way we could relate to. There are no external physical constants. There is only Will. What is 'real' becomes a plastic, flowing thing shaped by the Wills that inhabit the Void. These Wills consume, but nothing ever really dies. They're just drawn into the purpose of the stronger Will.
What would such beings see in a world like Runeterra?
"Such... hideous creatures..."
They would see a world calcified in form. Seared by burning light and suffocated by smothering law. A place where the pitiless, merciless march of unyielding physical reality destroys Wills. A place where Wills cease to be, where their rotted, festering remains are gorged upon by Nagaboros and vomited back out as mockeries scoured of the minds that once were. A place where things perish and cease to be. A place where Wills wither under the tyranny of unfeeling rules that serve only to perpetuate an endless cycle of purposeless suffering.
If any of the Wills that dwell in the Void are capable of feeling anything like empathy, how could they allow such a hideous, existentially terrifying place to continue existing when it's within the realm of their ability to end it?
And end it they will.
In the only way they know how.
"Om nom nom nom nom!"