In the DC Comic books, Mark Shaw was one of several characters to take over the Manhunter title, including Kate Spencer, a character played by Chelah Horsdal in Season 2 before she was killed off.
Arrow's version of Mark Shaw will be an A.R.G.U.S. operative in Corto Maltese, who crosses paths with Diggle (David Ramsey). Shaw also has connections to The Suicide Squad in the comics, a group of villains that was introduced in Season 2, when they all took on a mission for Amanda Waller (Cynthia Addai-Robinson).
David Cubitt joins a growing list of new cast members for Arrow Season 3, including Brandon Routh (Atom), Karl Yune (Maseo), Devon Aoki (Katana), J.R. Ramirez (Wildcat) and Charlotte Ross (Felicity Smoak's mother). We also reported last week that Ra's al Ghul will be the primary villain this season, although he has not been cast yet.