The Void Shadow was a creature that briefly appeared as part of the Inu the Crone plotline.
It physically resembled an Abyssmal Horror, but was far, far more powerful. To make matters worse, it spawned with several dozens of helpers, including Paragon Devourers of Souls.
The Void Shadow was located in a special dungeon, called the Void Shadow Dungeon. It was accessed by traveling to the eastern end of the Burned Forest in Ilshenar and speaking the word roclasmpy in a particular location.
There are unconfirmed reports that the Void Shadow dropped a Crimson Cincture as loot upon death.