Manu Bennett’s Slade Wilson was introduced in the Arrow Season 1 flashbacks as another occupant on the island where Oliver Queen was trapped.
In Season 2, viewers watched his descent to villainy in the flashbacks, while the present day storyline saw Slade exacting his vengeance on Oliver Queen after years of planning.
Slade taught Oliver how to fight and became a close ally.
Deathstroke hasn’t been seen on Arrow since then, but series star Stephen Amell recently suggested that Bennett’s iteration of the character could return without there being any conflict with the movie division.