In an interview with ComicBook.com Neal McDonough confirmed that his Arrow character Damain Dahrk would in fact appear in The Flash and Legends Of Tomorrow.
"You go back to the League -- I did an episode of Legends last week -- and I look exactly the same in 1975 but you see he's not as confident and doesn't have such an agenda forty years ago as he does now, so it's fun to play the different times, you know? Going back in time with Legends, and The Flash, with The Flash, I'm just this crazy, fun, enjoy everything while I kill everybody type of character. It's just nuts."
"I'm enjoying the power that I have because it's my time," he said of his character. "It's not Savage's time, it's not Ra's al Ghul's time; it's Damien Darhk's time."
(Green) Arrow and The Flash are set to crossover again in episode 8 of The Flash's 2nd season titled "Legends Of Yesterday". Legends Of Tomorrow is set to start airing sometime after the winter break.