Speaking to TV Insider, the show runner for all things DC on the CW, Marc Guggenheim, gave some story hints as to why Constantine will be appearing on Arrow, and that it has to do with the resurrection of the deceased Sara Lance. Fans know from teasers and trailers that Sara does eventually return seeing as how the White Canary is to join up with Legends Of Tomorrow early in 2016 and that her resurrection is brought upon through the use of the Lazarus Pit.
According to the site, Constantine’s arrival will “factor into the fallout” Sara’s resurrection. Said Guggenheim:
“Unlike Thea, who wasn’t fully dead, the consequences of using the pit on someone as dead as Sara was creates some unforeseen difficulties, and John Constantine is uniquely suited to help our team deal with those.”
Arrow returns on October 7 with new episodes airing Wednesdays at 8 p.m. ET/PT on The CW.