Legends of Tomorrow: Who are the Justice Society of America?

Tonight's episode of DC's Legends of Tomorrow, titled "Legendary," introduced a game-changing development in the final moments of the episode.

For about a month now, there have been reports that a caped hero would be introduced in tonight's episode, and that his mysterious agenda would drive much of the second season.

When we got to meet him, he arrived on board a trashed version of The Waverider, telling the team that Mick Rory had sent him to their time and place.

"I'm Rex Tyler," he said. "I'm a member of the Justice Society of America."

The Justice Society of America are DC Comics's first super-team. Originally featuring the Golden Age versions of Superman, Wonder Woman, Batman, The Flash, Green Lantern, and more, the team was reinvented after the introduction of the DC multiverse in the Silver Age. Then, they became Earth-2 versions of the classic characters. After the Crisis on Infinite Earths, the Earth-2 elements were removed and the JSA simply became a team that started in the '40s. At that point, there was no Superman or Batman, and members like Green Lantern, The Flash, Hourman, Sandman, and Hawkman were played up.

The team became a major force in the DC Universe, launching a number of legacy teams like the Young All-Stars and Infinity, Inc. -- the children and successors of the JSA. In the 21st Century, as the idea of "legacy" became important to DC Comics, JSA was one of the best-selling and best-reviewed books the publisher had.

Rex Tyler is Hourman, a superhero who has enhanced abilities for an hour at a time.

"Doing Hourman on the show is also super exciting to us," executive producer Marc Guggenheim told TV Insider. "But what really got our motors running was, it's not just Hourman, it's Hourman saying, 'I'm over there with the Justice Society of America.' We're going to do Justice Society of America next year."

Other characters with strong ties to the Justice Society, who have previously appeared on The CW's DC shows, include Mr. Terrific (Curtis Holt from Arrow), Atom-Smasher (a Season Two villain on The Flash) and Hawkman and Hawkgirl, who left Legends of Tomorrow with tonight's episode.