I just got a chill. Reactive: Additional FX after eliminating opponents.
Ghost Portal backpack is a Epic Fortnite Backpack from the Skull Squad set. Back bling Ghost Portal 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 Skull Squad contains 13 cosmetic items.
The backbling Ghost Portal belongs to Chapter 1 Season 5. It's not yet known how to get the backbling Ghost Portal.