Sarah Michelle Gellar joins the cast of Showtime's 'Dexter: Original Sin' in a special guest-starring role as Tanya Martin, the CSI Chief at the Miami Metro Police Department and Dexter's new boss. The prequel series follows a young Dexter transitioning into a vigilante serial killer with the guidance of his adoptive father Harry, played by Christian Slater. Gellar's addition complements a star-studded cast including Patrick Dempsey and Patrick Gibson. 'Dexter: Original Sin' is set to delve into Dexter's origins in 1991 Miami, detailing his journey from a student to a calculated killer while navigating his internship at the police department. The series, consisting of 10 episodes, does not have a confirmed release date yet.