It seems that a major villain from Arrow will be making his return later in Season 3.
Spoilers to follow.
When last Arrow fans saw Deathstroke, he was in an A.R.G.U.S. prison on the island of Lian Yu. He vowed to one day escape, but whether or not that day comes in Season 3 Episode 14 or not remains to be seen. Maybe so, or maybe we – or one of the other Arrow characters – just get to see him behind bars.
In the meantime, Arrow will have to deal with Barry Allen in this weeks' crossover, beginning with tonight's episode of The Flash, and continuing in tomorrow night's episode of Arrow.