The Catalyst is the name of one of the back bling cosmetic items for the game Fortnite Battle Royale. Just like all the other back blings, the Catalyst does not provide any advantage or have any gameplay effect. This back bling is a part of the Fortnite Starter Pack.
The Catalyst back bling features a black heavy backpack filled with advanced gizmos for agents and special forces. The backpack also features a long antenna from what seems to be an advanced transmitter protruding from one of its pockets. Other pockets of the backpack are also filled with some advanced gadgets. All items, including the backpack itself, does not provide any advantage in game, but is meant to be purely aesthetic.