Recently the 21st episode of Arrow; "Al Sah-him" teased the DC character Damien Darhk.
So who is Damien?
Ra's tells Oliver that Darhk joking the League around the same time as he did and that he tried to buy the earthquake machine from Malcolm Merlyn and also informed Mark Shaw to steal A.R.G.U.S. documents. He maybe tied up to previous events of the show because he is most likely to be the Arrow season 4 villain.
Damien is a character from the Teen Titans comics debuting in Teen Titans No. 1 of 1999 and he only existed in that 12 episode arc. He ended up being killed by Vandal Savage at the end of the arc but he hasn't appeared in the New 52 comics. At Wonder Con David Ramsey said that H.I.V.E. would play a major role in season 4 this will be easy to do since Damien is a major H.I.V.E. character.
Damien also gets tied up with Spade Wilson's wife Adeline Kane.
This would be a easy way for Manu Bennett to reprise his role as Deathstroke!