AceShowbiz - "Arrow" is apparently casting a role of a key villain for season 4. ComicBook.com has obtained transcripts for two actors auditioning for a role listed as "Simon," though it's believed that the name is either an alias or just a pseudonym.
In the dialogue, the villain reveals to Oliver his past connection with Ra's al Ghul. "I'm the man even Ra's al Ghul fears," so he claims. He also indicates that he's more dangerous than Slade Wilson and Malcolm Merlyn whom Oliver has dealt with.
The dialogue raises speculation that the character is actually Damien Darhk, Ra's al Ghul's former ally in the League of Assassins. In the comic books, Darhk believed he should be the one leading the League and was angry when Ra's was chosen instead. After escaping with his followers, Darhk leads an organization called H.I.V.E.