Regarding Event Skins

I really want the Sona skin that's part of the event.

The problem is, I'm not sure how to get it. Do I have to buy it or is there a way to earn it? The event page is long and convoluted and doesn't really explain the end dates very well (it says the skin is only available until the 24th but doesn't specify if that's just the RP purchase that ends or if it can still be earned with quests, which go into October).

Can I get some clarification on how to exactly earn this skin? Or does it have to be purchased? Don't want to miss my chance.