This may be an idiotic question...

Nobody has a god damn clue, and the things that probably do know probably aren't inclined to tell, if you could even comprehend them.

It might have been pre-existing like its own dimension but the lines between the dimensions have blurred in some places in rune terra , kinda like the fps shooter - DOOM ...but thats just my take on it

sense every thing in the void is apparently blind does that make s a void champ jk. possible void lee sin skin?

"The Void is a nightmarish dimension said to exist outside space and time, where terrifying living weapons were wrought to win an ancient and long-since ended war."

Who made the void is unknown, if it was made. The original denizens of the void were created as living weapons to win an ancient lore. Whether these are the same voidborn we see now, or they have undergone eons of evolution is unknown. It's easy to guess though that the reason they're there is that after the ancient war was won the winner said to themselves. "Oh man, what should I do with all of these horrifying creatures?!" Then they locked them up in the void and said "They shouldn't be able to get out of there".