WARNING: The following contains major spoilers for "Crisis on Infinite Earths," the latest Arrowverse crossover.

The CW's "Crisis on Infinite Earths" was packed with cameos, but "Part 5" included one that's particularly notable. In that eepisode, Crisis on Infinite Earths co-creator Marv Wolfman, who wrote the original comic book event, made a special appearance.

The DC's Legends of Tomorrow installment of the crossover picks up as Supergirl awakes in her apartment. She soon receives a call to stop a villain downtown, and arrives at to discover Weather Witch, an established Flash villain. The Scarlet Speedster himself swiftly follows, leading them to deduce that their Earths have merged. That's when Wolfman approaches them.

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"Supergirl? Flash? I just love it when you guys team up! Would you sign this for me please?" Wolfman asks, folder in hand.

"OK," Barry replies, puzzled. He takes the folder from Wolfman's hands and flips it open to find a photo of himself and Supergirl standing together.

 

"Wait, you know both of us?" Kara asks. "And it's normal to see us together?" Barry adds.

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"Well, normally, you'd also have Green Arrow and a Legend or two," Wolfman explains. "Last year, even Batwoman joined in." He points to the folder. "Would you make that out to Marv? Thank you!"

"You're welcome," Barry says as he scribbles. "Marv, as far as you know, how long have Supergirl and I and all the rest of us been working together on this Earth?"

"Uh, since forever!" Wolfman answers.

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That's the final nail for Barry and Kara, confirming that their Earths indeed merged to become Earth-Prime. It seems, when Oliver Queen restored the multiverse by stopping the Anti-Monitor at the dawn of time, he made it so the Arrowverse's heroes could live together in one world.

In addition to Crisis on Infinite Earths, Wolfman helped to create a slew of iconic superheroes, including Blade, Daredevil's Bullseye, Nova, Raven, Starfire and Cyborg. He also had career-defining runs on The New Teen Titans, The Amazing Spider-Man and Adventures of Superman, among others. In addition to appearing in "Part 5," he co-wrote "Part 4" with producer Marc Guggenheim.

"Crisis on Infinite Earths," the latest five-show crossover between all the Arrowverse series, will soon be available on The CW's streaming service in its entirety.

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