Yesterday fans learned of the "one-time-only deal" -- as explained by "Arrow" executive producer Marc Guggenheim -- that will bring Matt Ryan's John Constantine back to the small screen following NBC's cancellation of his own series. Constantine will appear in the fifth episode of "Arrow's" fourth season and, as Guggenheim hinted to TV Insider during the CW's Television Critics Association presentation, it will have major ramifications for both "Arrow" and the upcoming "Legends of Tomorrow" series.

Guggenheim hinted that Constantine will be brought in to deal with the fallout of the resurrection of Sara Lance (Caity Lotz) via Ra's al Ghul's Lazarus Pit. "Unlike Thea [Queen], who wasn't fully dead [before being resurrected by the Lazarus Pit], 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," said Guggenheim. Following her resurrection on "Arrow," Sara Lance will travel to the midseason CW series "Legends of Tomorrow," where she will take on the mantle of White Canary.

Guggenheim also expressed his enthusiasm for Constantine's addition to the "Arrow"-verse.

"We are super-crazy excited about this," said Guggenheim. "Each year on the show, we have added and expanded the mythology with different elements. In Season 2 we introduced the idea of superpowers, last year brought in the League of Assassins, and for Season 4, we always had a plan to introduce magic and mysticism [like Ra's al Ghul's Resurrection Pit]... We were already swimming in those waters and that's why Constantine is this great organic little development."

"Arrow" season four debuts on October 7.