The Sif Skin is a Marvel Fortnite Outfit. This cosmetic has been leaked, and the skin will hopefully be available to purchase in the near future!
Sif is another possible skin that could be available in Chapter 2 Season 4 of the Battle Pass. She appears in the comic book that is leading us into the season, which makes her a likely candidate to be one of the skins you can earn during the season.
How-to Get the Sif Skin
Sif was leaked or datamined, which means we don't know for sure how it will be released. Most cosmetics that are leaked will be released within 30 - 60 days of their discovery.