There have been whispers of an organization known as the Claw of Shar, however, any details of their existence and abilities known by them are merely rumor. If they do actually exist, this might have something to do with them. Or maybe not. There is no guarantee of the accuracy or validity of anything below.
Shadow Cloak is believed to be an ability utilized by the Claw of Shar.
By weaving nether into a shadowy cloak around themselves, an assassin be able to move unnoticed with greater ease in areas that are darker than normal.