Ben Affleck revealed test footage on Twitter this morning of Deathstroke, an assassin/mercenary who is the co-founder of Lex Luthor’s Secret Society of Super Villains in the DC universe. Deathstroke is also a nemesis of Batman’s, and at one point worked with Ra’s al Ghul’s daughter Talia. Together they controlled Robin via neural implant to kill Batman.
It’s not clear how Deathstroke will factor into Warner Bros’ DC Universe — whether the character will appear in the upcoming Justice League or Affleck’s Batman stand-alone film. Likely it’s Justice League, which is coming out November 17, 2017.