Colton Haynes who played Roy Harper/Red Arrow in previous seasons of Arrow, has announced his return to the Arrow set, along with an article showing when he will be returning.
— Entertainment Weekly (@EW) November 20, 2015
Episode 12 is set to air after the Winter Break, but here is what Executive Producer Marc Guggenheim had to say in the Entertainment Weekly interview:
“It was always our intention to have him return following his departure last season, but it was a matter of finding the right story line to pull it off. That opportunity presented itself in episode 412 and we can’t wait for the fans to see this reunion.”