Void Stiles refers to Stiles Stilinski during the second half of Teen Wolf Season 3. Fans began using the term to delineate between the primary, "good" character and the "evil", Nogitsune-possesed version of Stiles. The writers of Teen Wolf incorporated the term into the show during Season 5.
After "dying" during a symbolic sacrifice to save his father, Stiles' soul is left vulnerable to influence by an ancient Japanese spirit. He is possessed by the Nogitsune and wreaks havoc on his friends.
Thanks to an intervention by Scott McCall and Lydia Martin, Stiles is able to escape the void spirit's grasp. They separate into two distinct beings, Void Stiles and Stiles. The original is left severely weakened as a result.
Stiles returns to normal after Scott destroys Void Stiles with a bite.
Main Article: Stiles Stilinski (Season 3)