The rarity of Flightless Fury is Mythical, and been equipped it takes a “Bundle” slot. At this moment the item is available for trade.
Flightless Fury is a skin for Vengeful Spirit. The skin was added to the game on October 16, 2015, it was created by ChiZ.
Flightless Fury has been recently added to the game, which makes it rather popular with the game community.