Deathstoke, aka Slade Wilson, was created back in 1980 created by Marv Wolfman and George Perez. The character was originally a primary villain of the Teen Titans, but has since been made to be a villain of Batman as well as Green Arrow. Deathstroke is an assassin and mercenary, who was given powers by a secret experimental serum. Powers for Deathstroke include superhuman strength, agility and an expert at martial arts.
Deathstroke was most recently played by Manu Bennett on Arrow, but the character was taken off the air because WB wanted Deathstroke for another project, which now looks to be either Justice League or The Batman (or both).
The inclusion of Deathstroke in the DCEU may even suggest Nightwing and the Titans exist. Interestingly enough, a live action Titans TV series was recentlys scrapped. Maybe to make way for the Titans in the DCEU as well? Geoff Johns did confirm huge plans are in store for the Titans.
"Justice League" has a November 17, 2017 release directed by Zack Snyder starring Ben Affleck as Batman, Henry Cavill as Superman, Gal Gadot as Wonder Woman, Jason Momoa as Aquaman, Ezra Miller as The Flash, Ray Fisher as Cyborg, J.K. Simmons as Commissioner Gordon and Willem Dafoe as an Atlantean.