The Shadow Chest is a limited edition chance chest. There are two Shadow Chests.
Old Shadow ChestEdit
Available between Aug 29 and Sept 3. Like all the limited edition chests it costs 30 gems each or 300 for 11 chests.
The shadow chest has 4X chance for legendary, and all rewards are Super rare (**) or higher.
Unfortunately it does not seem like it can give the Cloudrange Platemail epic armor.
New Shadow ChestEdit
Available from March 7 2014. It costs 30 gems every chest.
The new epic introduced in it is the Arcane Mantle
There's a 3x chance for an epic or a legendary