Arrow announced some big news earlier today about season 4. The big bad has been cast. We've heard early reports that it would be Damien Darhk, but up until now the character has not actually appeared. Damien Darhk will be played by legendary character actor Neal McDonough. McDonough has a lot of experience playing cold-blooded villains, such as his roles in, Justified, Red 2, and Terriers. People interested in comic book media might recognize him as Dum Dum Dugan in Captain America: The First Avenger and Deadshot in Batman: Assault on Arkham.
For anyone unfamiliar with the character Damien Darhk, in the comic books he is an old Teen Titans|Teen Titans villain. He originally appeared as a figure involved with the organization H.I.V.E.. Darhk has been introduced as the leader of H.I.V.E. on Arrow, making him a rival to Ra's al Ghul and the League of Assassins. This version of Damien Darhk seems to be combined with elements of another villain called Doctor Darrk. In the comics, Darrk has a similar rivalry with Ra's al Ghul and vies with him for control of the League of Assassins.