Crazy theory, you say? The Darkin, the Void, and a white horseman walks into a bar...

Since Aatrox the sword is war-themed and Rhaast the scythe is death-themed I'm gonna make the assumption, for this argument, that the Darkin are themed after the four horsemen; Conquest/Pestilence, War(Aatrox), Famine, and Death(Rhaast).

Notice Conquest/Pestilence; the rider of the white horse is sometimes interpreted as the former, sometimes as the latter.

Now, the nature of the Darkin is that they are magical living weapons of immense apocalyptic power that corrupt it's host and transform their bodies into a demonic, organic armor. I've heard this one before... the Void..?

From the Universe site:

The Void (...) where terrifying living weapons were wrought to win an ancient and long-since ended war.

The thematic and visual parallels are many and obvious, if one thinks about it.

My theory is that the Void was created by one of the Darkin run amock, trapped in another dimension to protect Runeterra or some other world(s).

As "a horseman": the Voidborn are hungry hungry hippos, so Famine would make sense. My personal favorite theory, though, is the Void representing the white horseman, Conquest AND Pestilence, a combination of their nature as conquering, world-devouring weapons and insectoid horrors. Bonus connection: when voidborn first enter runeterra they start out as ivory white, like the horse, according to Diana's Night's Work story.

So that's my theory! Of course, something more down to earth would be Varus' weapon being Conquest, the weapon of the white horseman being a bow n' all. But that wouldn't make for fun n' crazy theory-crafting!