The Shadow Assassin is a mysterious bounty hunter who was hired to kill Matrak in the Third Movie. He has many weapons at his disposal, as shown in the animation. A pair of blasters, a net gun, and a sword. As shown in the second episode of the movie, the sword can project a laser covering to increase its power.
He appears to be one of the Dark Hunters, being a bounty hunter and having many weapons meant to serve this purpose. He is obviously very skilled, having been shown capturing or killing a whole team of toa.
As he is on the poster, he will probably be offering more of his services to the villains.