This game mode is maybe confirming what I said on a previous thread? (about Star Guardians/the Void)
Unfortunately that thread got forgotten and buried in the middle of various "Why Ahri/Miss Fortune is getting another skin" threads, so I am going to post another new one.
The Void IS the enemy from the Star Guardians. They are fighting to prevent a Void invasion. It is sad that the Star Guardian skins are not canon because that would be a cool thing to add to the lore.
Until a rioter confirms that I cannot 100% claim that this is true, but everything seems to indicate that this is true. Either that or this mode is just reusing the Void models for alien monsters.