The hidden Snowfall skin was revealed earlier today to be The Prisoner, a humanoid creature that was bound by chains at Polar Peak.
Players that have earned The Prisoner and its corresponding backbling, Padlock, can unlock a second stage of the skin. Although they will need to equip both The Prisoner and Padlock, or they will not be able to unlock the next stage, it's fairly simple.
Once you land at the Polar Peak castle, you will need to run down the stairs to the ground level. There you will find a table next to several bookshelves. On the table, there will be a floating key, which you can interact with. Once you do that, the ice crystals that covered The Prisoner's shoulders and skull instantly melt, revealing more of its grey skin.
Several more stages are expected to make their way into the game, but it will probably take another week before the third stage pops up.
If you still have not unlocked The Prisoner and Padlock, you will need to complete 60 of the challenges available in this season’s battle pass. Without the battle pass, there is no way to earn The Prisoner.
It is likely The Prisoner or something else at Polar Peak will be the catalyst for change as Fortnite heads into its eighth season in a few weeks, although it's unclear what the event will be to change the map or the game's items.