Robin Williams is known for his humor, but that won't be the case when he guest stars on an upcoming episode of Law & Order: Special Victims Unit.

Williams will play a Svengali-like figure who tests Stabler and Benson, according to TVGuide.com. A series spokesperson describes Williams' character as an "engineer whose life has gone terribly wrong" and who faces serious repercussions. The episode will air April 29.