The latest trailer for The CW's upcoming "Crisis on Infinite Earths" event, seen above, gave fans a first look at a character played by Wil Wheaton and a fully-realized tribute to "The Death of Superman" storyline. It also included a phrase that is raising some eyebrows among fans: "Join the League." While Superman, The Martian Manhunter, The Flash, Green Arrow, and numerous other major Arrowverse characters have a long history with the Justice League in the comics, The CW has thus far been unable to use the name, likely due to entanglements related to the movie. The Justice League is regularly referenced on DC Universe's Titans and Doom Patrol, but more as a cultural touchstone rather than as characters we might see in the flesh.
So what -- if anything -- changed? Well, assuming there's anything actually TO the Arrowverse heroes calling themselves a "League," it could be a few things.
First of all, the failure of the Justice League movie and the slow process of rebuilding DC's cinematic universe likely means that the theatrical part of Warner Bros. doesn't have any use for the name anytime soon. Yeah, there's a persistent movement to release Zack Snyder's directors' cut of Justice League, but even if that were to happen it would not change the trajectory of the DC Films universe (which at this point is actually a multiverse, since movies like Joker clearly don't take place in the same universe as Shazam! and Man of Steel).
It's also the kind of thing that would make sense coming out of "Crisis on Infinite Earths." In the comics -- especially the post-Crisis on Infinite Earths reinvented comics -- the League forms to tackle challenges that no one hero can handle on their own. If there has ever been such a problem, it's the Crisis -- which required the assistance of heroes from other Earths which might or might not survive the event -- that might key such an instinct off in the heroes.
Additionally, "Crisis" showrunner Marc Guggenheim has been teasing a big reveal prior to Sunday's "Crisis" premiere...and it's hard to imagine one more seismic than the foundation of a Justice League in the Arrowverse.
"Crisis on Infinite Earths" has loomed over the Arrowverse for years. The series premiere of The Flash featured an allusion to the hero disappearing amid red skies in the year 2024. During the original comics event, red skies were a sign of doom to come to a world during the Crisis. At the end of last season, a couple of things happened: The Monitor (LaMonica Garrett), who had appeared in "Elseworlds," reappeared and revealed that Oliver Queen was destined to die in the Crisis...and the future newspaper at STAR Labs rolled back the expected date of the Crisis from 2024 to 2019.
In the comics, Crisis on Infinite Earths centered on a battle between the combined superheroes (and even some villains) of the DC multiverse and an immortal, cosmic threat known as the Anti-Monitor. Like The Monitor, the Anti-Monitor will be played by LaMonica Garrett in the Arrowverse. As the Anti-Monitor destroys realities, he replaces their positive matter energy with antimatter, growing his own power and sphere of influence. He was eventually stopped by the sacrifices of several heroes, including The Flash and Supergirl, as well as the merging of multiple universes to save reality by becoming a single, unified timeline. Fans have long wondered whether the events of "Crisis on Infinite Earths" might bring Supergirl and even Black Lightning to Earth-1, where the rest of the series take place. The crossover will also feature guest appearances by Tom Welling as Clark Kent, Erica Durance as Lois Lane, John Wesley Shipp as the Flash of Earth-90, Johnathon Schaech as Jonah Hex, Kevin Conroy as Bruce Wayne, and Ashley Scott as The Huntress.
“Crisis on Infinite Earths” kicks off on Sunday, December 8 on Supergirl, runs through a Monday episode of Batwoman and that Tuesday’s episode of The Flash. That will be the midseason cliffhanger, as the shows go on hiatus for the holidays and return on January 14 to finish out the event with the midseason premiere of Arrow and a "special episode" of DC's Legends of Tomorrow, which launches as a midseason series this year and so will not have an episode on the air before the Crisis.