Serenity's Summer Glau is already on board
Arrow season 2, and now THR report that her screen brother Sean Maher has also signed on as the evil Mark Scheffer, better known as DC supervillain Shrapnel. In the comics Shrapnel is made of metal and has superhuman strength and stamina, but it's unlikely that this will carry over to the show -- although he does share some of the same traits as his comic counterpart, as THR reckon he'll be a "serial bomber who places Starling City in a grip of terror and threatens the life of Sebastian Blood (Kevin Alejandro)". Maher will guest star in the tenth episode of the CW show.