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Sean Maher, who co-stared as Dr. Simon Tam on Joss Whedon's Firefly, is set to make a guest appearance on the second season of the CW's Arrow . The actor will play Mark Scheffer/DC Comics supervillain Shrapnel beginning on the 10th episode. This is the second Firefly alum to make their way onto Arrow, with Summer Glau, who played Maher's sister River on Whedon's space-cowboy series, also cast in a recurring role on the CW superhero show. THR reports that Scheffer will enter the scene as a ruthless serial bomber who terrorizes Starling City and puts Sebastian Blood's (Kevin Alejandro) life in jeopardy. In the comics Shrapnel is made of metal and possesses superhuman strength. Black Canary, Bronze Tiger, Flash, Sin, Isabel Rochev and the aforementioned Sebastian Blood are all set to make appearances this season.