Mjolnir is a Marvel Fortnite Harvesting Tool from the Thor set. Mjolnir was available via the Battle Pass during Season 14 and could be unlocked at Tier 8.
One of the unique things about the Mjolnir Pickaxe is that only the Thor and Captain America outfits can wield it. This stays fairly true to the comic books, although it looks like some future heroes will be able to possibly use it as well.
How-to Get the Mjolnir Harvesting Tool
Mjolnir is part of Season 14. If that season is still currently in the game, you can obtain this item by purchasing and/or leveling up your Battle Pass.
Mjolnir is part of the Thor set! You can find all of the items that belong to it below.