The Way of the Shadow is the new Assassin specialization for Blade & Soul. Here are the details.
A new stance is introduced, where some abilities may seem familiar to you. However, other abilities have been updated. Phantom Marks and stacking them are set to be the main mechanic in the specialization. Stacking five of them will allow you to use special abilities.
Once you’re in the Phantom Stance, you’ll be able to use Phantom Shuriken and Night Reaver. To use these, you’ll need Phantom Mark stacks. However, any abilities which use up your Phantom Stacks give you stacks of Twilight. These Twilight stacks can be used to for Twilight Strike.
You can learn more about the Way of the Shadow here.