Warning! This article contains SPOILERS for Teen Wolf: The Movie.Teen Wolf: The Movie finally provides some clarity for a mystery left unanswered in the third season of Teen Wolf regarding its villain for the season. Teen Wolf: The Movie is set more than 10 years after the ending of Teen Wolf and, thus, features many of its original characters as now adults who return to Beacon Hills to defend the Californian town from a supernatural threat that is at the same time old and new. In fact, Teen Wolf: The Movie borrows some of its plotline and villains from Teen Wolf season 3.
https://screenrant.com/teen-wolf-movie-adrian-harris-season-3-villain-return/