Shadowfell

Until recently the domain of the Raven Queen, the Shadowfell is a plane closely abutting the Material, a dark and twisted version of the world. Though not strictly evil, it is corruptive, tainting those who wander its gloomy trails for too long, and even the material world where the two overlap slightly, such as at rifts between the two.

According to Kalarel's Journal, Tiamat's soul has found access to the Shadowfell from her prison realm, usurped the Raven Queen in some fashion and dominated the Shadowfell as her own domain, darkening it further and attempting to increase its influence on the real world, in hopes of someday being able to take form on the Material again to take revenge on Bahamut and the Platinum Empire. Additionally, she has been bolstering her forces with the denizens of shadow, and has even hinted to Kalarel that she found new varieties of dragon in the gloom that have joined with her.