It was America’s birthday, so it makes sense that Captain America might show up at Fortnite. According to the video, it arrives via Bifrost. The first Avenger is now available in the game store — or rather, its equipment and two items are available.
Right now, all players can pick up Cap’s equipment, along with his Proto-adamantium pick (cap’s shield) and back trinket. There is also a large Emot salute that includes many fireworks. You can see the emotion in action on the Fortnite site.