The rarity of Obsidian Nightmare is Rare, and been equipped it takes a “Bundle” slot. At this moment the item is available for trade.
Obsidian Nightmare is a skin for Queen of Pain. The skin was added to the game on March 16, 2016, it was created by Toasty.
Obsidian Nightmare has been recently added to the game, which makes it rather popular with the game community.