The Oscar winner will play a Svengali-like figure who tests Benson and Stabler like never before.
Exec producer Neal Baer says the episode is "one of the best cat and mouse stories we've ever done," and describes Williams' character as "a guy who has a problem with authority figures." Adds Mariska Hargitay: "He is one of these charismatic human beings that has uncanny power over people to get them to do what he wants."