Wind her up. See what happens.
Harley Quinn's Revenge backpack is a Fortnite Backpack from the ...And Red All Over set. This back bling is currently unreleased. It's an upcoming backbling that have been leaked but hasn't been put on sale in the Fortnite Item Shop yet! Back bling Harley Quinn's Revenge is reactive. It means it changes or evolves (reacts) depending on certain things in game (damage dealt to opponents, if it's day or night...)
The Set ...And Red All Over contains 1 cosmetic items.
The backbling Harley Quinn's Revenge belongs to Chapter 2 Season 6. This backbling is a leaked backbling, an upcoming item that will release in the Fortnite Battle Royale game. Therefore, you cannot buy it yet at the Fortnite Shop or get it by any other method.