The 2017 Arrow-Flash TV crossover across The CW/DC series including Arrow, The Flash, Supergirl and Legends Of Tomorrow is set to revolve around the heroes facing their own evil versions from Earth-X, a world where Nazis won World War II.

The actors are currently filming the epic, four-episode crossover event, and leaked set photos have revealed some more details, pertaining to the return of a long lost Flash villain.

[Warning: Spoilers Ahead]

The photos shared on Twitter features actor Tom Cavanagh wearing the Reverse Flash suit, with the hood pulled down away from his face. The suit matches that of the Earth-X Flash on the poster, with the yellow chest turning into a black suit below the waist.

The images also show Grant Gustin in his red Flash costume walking next to Cavanagh (wearing only part of his costume).

Crisis on Earth-X is the official title of the 4 part crossover. In Earth-X, Germany won the World War II and Green Arrow, The Flash and Supergirl all are evils. The event will air over two nights from 27 November to 28 November across The CW shows, and the network has released an official poster featuring all the event.

Here's official logline for the crossover: With all of the heroes in town to celebrate Barry and Iris' wedding, villains from Earth-X crash the festivities with a deadly agenda.

Executive producers Marc Guggenheim and Andrew Kreisberg opened up about the 4-show crossover event to Deadline. They said, "We conceived this year's crossover to be evocative of the annual Justice League/Justice Society crossovers we grew up with and looked forward to as kids. Phil was the perfect choice to bring the comic book cover iteration of this idea to life."

"You can count on one hand the number of artists who could compellingly represent so many characters in a single image. Phil is one of them. And his design invokes the cover ofJustice League #207, which was the 20th annual JLA/JSA crossover. Our 10-year-old selves are in heaven," the executive producer was quoted as saying by Deadline.

Arrowverse crossover poster
The official poster for Arrowverse crossover titled, Crisis on Earth-X The CW/ DC TV