Lorecraftin' time! What and where is the Void?
Title asks the question and here's my personal answer: It's post-apocolyptic Nasus/Renekton Realm.
Zilean's experiments cracked the temporal plane. Meaning his meddling can allow summoners to pull not only from different realms but different timelines within those realms as well. Nasus/Renekton's realm made a habit of creating animalistic guardians that both led and protected them yes? So what would happen if Nasus and Renekton's vacation in another dimension sparked a war amongst the rest of the guardians? The sudden power vacuum of what was probably the two strongest guardians could easily do this, especially seeing (through Renekton) that these guardians are not paragons of virtue at times.
So a war starts. More guardians are needed, faster, better, and with much less regard for aesthetics. Thus, the first void creatures are born. But they are merely weapons with the approaching danger of self-assembly. Enter in the classic sci-fi concept of the singularity, and the void creatures take over. Maybe with a hive mind super void creature? Or simply by consuming anything that isn't void? Imagination can go wild there, but what it does mean is the war ends with the Void creatures winning and not the original inhabitants. So what else is there to do but create more and more powerful void creatures and send them to conquer, consume and dissect to perfect their creations?
Thoughts? Your own theory?