The rarity of Apostle of Decay is Mythical, and been equipped it takes a “Bundle” slot. At this moment the item is available for trade.
Apostle of Decay is a skin for Necrophos. The skin was added to the game on June 2, 2015, it was created by motenai.
Apostle of Decay has been recently added to the game, which makes it rather popular with the game community.