The Star Guardians are all killed, which prompts Dark Star Thresh and all his cronies to wipe out all life on Runeterra. They almost manage to completely send Valoran into the stone age but before the last of those that have lived are destroyed by the Dark Stars, Arclight Yorick and all the other subsequent Arclight themed champions swoop in to protect Runeterra, saving some if not all life-forms from impeding doom.
They managed to send Thresh into another dimension to trap him, but only for a temporary amount of time.
The Arclight's know that Dark Star Thresh will return, even more powerful than before. In order to prepare Runeterra for another wave of destruction from Dark Star Thresh, Arclight Yorick and the Lady use all of their powers to revive enlightened Runeterra denizens such as Heimerdinger, Ziggs and the scientists responsible for PROJECT in order for them to use techmaturgy to develop capable fighters to defend against Dark Star once more.
Meanwhile PROGRAM is developed to help heal runeterra-denizens from the destruction from Dark Star Thresh's and defend from future destruction.