DC Entertainment President and chief creative officer Geoff Johns let slip the casting information in an interview, though he refused to comment on whether Deathstroke will first show up in next year's Justice League before causing problems for Affleck's Bruce Wayne in the promised Batman outing.
Created by Marv Wolfman and George Perez, Deathstroke first appeared in Teen Titans #2 in 1980. Also known as Slade Wilson, Deathstroke is an assassin and a mercenary, and while he is more often found in the Green Arrow's rogue's gallery – and has turned up on the DC Entertainment TV series – he has crossed Batman's path several times. Their memorable first meeting was in a 1991 Deathstroke comic story arc titled City Of Assassins, where he beat the Dark Knight to a pulp.
The Batman standalone doesn't have an official schedule slot as yet, but Justice League is heading our way on 17 November next year.