The rarity of Dark Reservoir is Mythical, and been equipped it takes a “Bundle” slot. At this moment the item is available for trade.
Dark Reservoir is a skin for Wraith King. The skin was added to the game on November 29, 2017, it was created by katzeimsack.
Dark Reservoir is currently one of the most popular skins, but it is rarely seen in the game due to its high price.