Each of Fortnite's seasons since chapter 1 season 3 have had hidden skins that players could unlock by completing challenges. Normally this skin remains a mystery until around week seven or eight, but this time around, Deadpool - in typical Deadpool fashion - let us know that he was the secret right away, by swimming in a pile of V-bucks on the battle pass screen and trailer.
There's no official information yet on when the Deadpool skin will actually be unlockable, or how many challenges players will have to complete to unlock it. But we should learn more as Fortnite season 2 progresses.