ARROW has cast Nick Tarabay, best known as Ashur on SPARTACUS, as the sinister Digger Harkness aka Captain Boomerang in a multi-episode guest arc.
In the original comics, Harkness is a villain that has his sights set on defeating The Flash more than The Arrow, but his involvement in the Suicide Squad as likely what landed him in ARROW’s storyline (with potential for crossover!)
According to Entertainment Weekly, his role on ARROW will be slightly different:
On Arrow, Harkness will be a former A.R.G.U.S operative, highly skilled in martial arts and espionage, with a deadly thirst for vengeance against his former employers.