Development "Crisis on Infinite Earths" was first hinted at in the
Arrowverse in
the pilot episode of
The Flash. In September 2018, three months before the release of "
Elseworlds" (the
Arrowverse crossover in the
2018–19 television season),
Legends of Tomorrow executive producer
Phil Klemmer said that the next crossover had "a loose shape". In early December 2018, a week before the airing of "Elseworlds",
Arrow showrunner Beth Schwartz said that the producers of all the series "already know a lot about what's going on in next year's crossover"; according to
Marc Guggenheim, "Elseworlds" would "lay the groundwork" for it. The end of "Elseworlds" revealed the crossover title and premise as "Crisis on Infinite Earths", adapting the story from the 1985
comic of the same name by
Marv Wolfman and
George Pérez. "Elseworlds" introduced the characters
Mar Novu / Monitor and
Psycho-Pirate, and alluded to an impending "crisis". In January 2019,
The Flash showrunner
Todd Helbing said that the remainder of the
fifth season would contain elements of the upcoming crossover.
The CW president
Mark Pedowitz called the crossover "the biggest, the most complicated one" to date, and was hopeful that
Legends of Tomorrow (which did not participate in "Elseworlds" because of production complications) would be part of "Crisis". In May 2019, Pedowitz announced at The CW's
upfront presentation that the crossover would include
The Flash,
Arrow,
Supergirl,
Legends of Tomorrow, and
Batwoman in a five-episode event spanning late 2019 and early 2020. The crossover occurred in the ninth episodes of
Supergirls fifth season,
Batwomans first season, and
The Flashs sixth season, in the eighth episode of
Arrows eighth season, and as a special episode of
Legends of Tomorrows fifth season which had not yet begun airing; this episode is not considered the series'
season premiere. Guggenheim said that when they first presented the crossover to the studio and network, the creators stressed that they did not want the event to be "Crisis on CW Earths" but "to touch as many strands of the DC tapestry as possible". About the crossover's comparison to the
Marvel Cinematic Universe (MCU) film
Avengers: Endgame (2019), Guggenheim said "for
Barry and
Oliver, there is an emotional denouement that is reminiscent of
Endgame ...
Endgame is an exclamation point. 'Crisis' is a semicolon".
Lead-up The May 2019
season finales of
The Flash,
Arrow,
Supergirl, and
Legends of Tomorrow contained hints of the upcoming crossover, with the Monitor appearing in all series except
The Flash.
The Flash season six showrunner Eric Wallace said that when all the series' showrunners discussed plans for their series before the crossover, "all of this synergy started to happen" with "a lot of cross collaboration" in service to the crossover's events. Wallace felt the themes of grief, death, and the end of all worlds in "Crisis" worked with the story they were trying to tell in the first half of season six of
The Flash, particularly that of
Ramsey Rosso / Bloodwork, "who is facing the end of his own world". This allowed the season's start to help set up the crossover. Most of
Arrows eighth season is a prelude to the crossover. The
Arrow writers consulted with their counterparts on
The Flash about the destruction of
Earth-2 in "
Starling City" to ensure that they could destroy this Earth. On
Supergirl, co-showrunner
Robert Rovner said that the season would tease the crossover "in our way, which is a little bit different from the way
Arrow and
The Flash are doing it". Rovner cited the return of
Malefic (
J'onn J'onzz's brother) as a major plot thread leading to the crossover, calling it "a gauntlet for J'onn".
LaMonica Garrett, who portrays the Monitor and the
Anti-Monitor, said before the crossover that the Anti-Monitor's "presence [would] be felt before you physically see him ... You see just enough of him to know this guy means business, but it's not in your face in every scene leading up to" the crossover. The preceding episodes of
Batwoman,
Supergirl,
The Flash, and
Arrow contained a
tag scene of
Nash Wells accessing a chamber under Central City.
Tie-ins The
Black Lightning episode "
The Book of Resistance: Chapter Four: Earth Crisis" tied in to "Crisis on Infinite Earths" before
Jefferson Pierce first appeared in the crossover's
Flash episode. In the
Black Lightning episode, a red sky covers Freeland before anti-matter strikes
Jennifer Pierce and sends her to a void where she finds her Earth-1 and 2 counterparts. An anti-matter wave wipes them and her Earth out, except for Jefferson, who is transported to an unknown location. The crossover story was expanded in a two-issue comics series
Crisis on Infinite Earths Giant, written by Marv Wolfman (who wrote the original 1985 series) and Guggenheim, and illustrated by Tom Derenick, Trevor Scott,
John Kalisz, Andy Owens, Hi-Fi,
Tom Grummett, Danny Miki, and
Chris Sotomayor. The series reprinted material from the original
Crisis on Infinite Earths comic, and its second issue included material from
DC Universe: Legacies #6 (2010). About the series, Guggenheim said that its concept "came pretty early on in the process" when it was realized that the story's scope would exceed the five episodes. By creating the comics, it allowed the producers to utilize "all of the characters that we don't have in the crossover proper for logistical reasons, or financial reasons, or creative reasons... [The comic series] allows us to tell a major piece of the story that we're designing, with characters and concepts that we couldn't achieve in live -action". Its four main characters are
Felicity Smoak, the
Ray,
Nyssa al Ghul, and
Wally West, in addition to appearances by
Atom,
Sara Lance,
Batwoman, the Flash, the Monitor,
Lex Luthor, and others. Serious consideration was given to the
Huntress, but Guggenheim opted for Nyssa al Ghul because "we had done
a comic-book tie-in that bridged
seasons two and
three of
Arrow and [Huntress] had featured prominently in that, so Nyssa had not yet had her chance to be immortalized that way". Wolfman added that the comics were "created to tie in perfectly with The CW shows" so it felt like a main part of the story and not ancillary: "This is integral to the entire storyline". The first comic debuted at
Walmart on December 15, 2019, with the second releasing on January 19, 2020; both were released in comic stores the following month. The series is set during the first part of the crossover's
Batwoman episode. Its stories and behind-the-scenes extra content was published in a deluxe hardcover edition on July 7, 2020.
Writing was the overall creator for "Crisis on Infinite Earths"|alt=A photograph of Marc Guggenheim speaking at a convention The
Supergirl episode was written by Derek Simon and
Jay Faerber, with
Robert Rovner and Guggenheim contributing to the story; Don Whitehead and Holly Henderson wrote the
Batwoman episode; and Lauren Certo and Sterling Gates wrote
The Flash episode, based on a story by Eric Wallace. The
Arrow episode was co-written by Wolfman and Guggenheim, and the
Legends of Tomorrow episode was written by
Keto Shimizu and Ubah Mohamed. Guggenheim was initially nervous about collaborating with Wolfman due to him being one of his idols and writing with a "heavy hand", but thankfully, their collaboration had no problems, with Guggenheim appreciating Wolfman's work for Luthor in the crossover. The writers gathered for a two-week session to work on the crossover. Guggenheim, as the overall creative for "Crisis on Infinite Earths", began by creating "a series of big tentpole beats that could happen in each of the five hours". These moments were designed to be modular and could be moved throughout the episodes depending on each series' creative needs, actor availability, and other factors, while also determining episode order. Guggenheim's initial template had
Arrow as the fourth hour and
Legends of Tomorrow the fifth, "because in crossovers past,
Legends has always been the finale of the crossover". The CW requested that
The Flash be the third hour and after working "out those tentpoles, for a variety of different plot reasons, it made a lot of sense for the first hour to be
Supergirl".
Batwoman was the second hour. Certain characters were considered "integral" while others would make
cameo appearances, depending on actor availability. With the large moments determined, the writers separated to write each episode. The group reconvened to review the whole, making adjustments as necessary. Each series' writers would work on its title character's scenes, to ensure consistency. For scenes set in
Gotham City,
Batwoman showrunner
Caroline Dries retained the series' grounded tone because it was "not normal for [the characters of
Batwoman] to interact with aliens and travel to different universes". Although the crossover is considered "a five-hour epic", the title character of each series and characters appropriate in those worlds (such as
Barry Allen of
Earth-90 appearing on
The Flash) are highlighted. The episode scripts were completed by early September 2019. The original comic was created to help with
DC Comics' continuity problems that could intimidate new readers; since the Arrowverse does not have those, however, the crossover allowed its creators to focus on the characters and their interactions.
Supergirl and Batwoman's relationship was developed, after it was seen at the end of "Elseworlds" that "those two characters [are] supporting each other much in the same way that Barry and Oliver have been supporting each other in previous crossovers". This version of the Anti-Monitor was inspired by the character's appearance in the
New 52 "Darkseid Wars" storyline.
Sara Lance is the focus of the
Legends of Tomorrow characters, with the crossover "kick[ing] off Sara's emotional journey for season 5 of
Legends" and giving Sara "a different perspective on things". Guggenheim said that the crossover would try to incorporate "seminal moments" from the comic book, opening with a similar destruction of parallel universes. Regarding the deaths of
Supergirl and the
Flash in the comic storyline, Wallace conceded that the writers "can't just kill all of our number ones on our shows", but a proper balance was found in killing some. Although it was revealed in "Elseworlds" that Oliver would die in the crossover (which, Guggenheim felt, "
spoiled our own story"), his death in the first hour was an attempt to "surprise the audience" who may have expected him to die in the fifth; it also "establish[ed] the stakes going into the next four hours". The third-hour death of Barry Allen from Earth-90 was "a way to honor what was said [by] the Monitor ... saying the Flash must die", without killing
Grant Gustin's character. According to Wallace, this gave "it a nice story twist, [to] get in some emotion" while "complet[ing] a 30-year
arc" for
John Wesley Shipp's character, who felt thankful in turn as his show didn't get a proper
series finale. The
flashback scene from the
1990 Flash television series, featuring Shipp's Barry Allen with
Amanda Pays' Dr. Tina McGee, was added while editing the episode "to give a little bit of his life flashing before his eyes". Oliver dies a second time at the end of the
Arrow episode. Guggenheim said that it was also done to "[surprise] the audience with the unexpected" ... "I think
[Stephen Amell] enjoyed A) the opportunity to play two death scenes and B) the creative sleight of hand in terms of surprising the audience". The inclusion of
Clark Kent and
Lois Lane from
Smallville in the crossover explored the characters after
Smallville ended.
Smallville Season Eleven, a 2012–2015 comic book series, had previously explored Clark's adventures after the show's
tenth and final season, and Guggenheim confirmed that while not directly referenced, the events from the comic series were considered
canonical for the crossover.
Erica Durance, who portrayed that series' Lois Lane, said: "It's a little bit of a look into their future and the different choices they've made to be together. It's a full-circle moment that's really nice". The
Batwoman episode in which the characters appear was written by former
Smallville writers, allowing Guggenheim to "[step] aside and let them speak to where things were headed. As a fan of the show, it answered a lot of questions that I had. It provided a lot of closure, I think, in a really nice way". The removal of this Clark's powers was a reference to
Superman II (1980), where
Christopher Reeve's
Superman temporarily surrenders his powers. Although the details of how
Smallville Clark lost his powers are not explored, "the idea that Clark would give up his powers in order to have this kind of life, that really resonated with all of" the writers and
Tom Welling.
Daily Planet articles written by Lois were shown "to make it very clear" to the audience "that he did become Superman, that he did have all these adventures, but they are left to your imagination".
Smallville co-creator
Alfred Gough was informed about the
Smallville scenes before filming began. The three actors who portrayed Clark Kent / Superman in the crossover did not have a scene written containing all three. The writers wanted a clear distinction between Welling's character at the Kent Farm and
Brandon Routh's character at the
Daily Planet. Guggenheim said, "The desire to have multiple Supermen [together] is what inspired the backup story" in the comic tie-in. Appearing as
Bruce Wayne in live-action for the first time allowed
Kevin Conroy to "explore a lot of [the] dark corners" of the character in a different way than when he voiced an older version of the character in the
DC Animated Universe (DCAU) series
Batman Beyond.
Lucifer Morningstar's appearance was considered to be five years before the events of
Lucifer, according to actor
Tom Ellis: "This is him in his proper playboy, don't-give-a-damn-about-anything stage in his life. He is very irreverent with our characters when they turn up in the scene and obviously, with
Constantine, there is some history there". His scene, written by Certo (a
Lucifer fan), was sent to that series' creative team for their feedback.
Lucifer co-showrunner Joe Henderson suggested Lucifer's flirting joke, and Ellis contributed his pronunciation of Constantine as "Constantyne". Henderson called Lucifer's appearance "the coolest thing" since he had wanted to participate in the crossover when he learned that it was adapting
Crisis on Infinite Earths. Black Lightning's inclusion was decided "very late in the game", according to Guggenheim. He worked closely with
Black Lightning showrunner
Salim Akil to incorporate the character not as a cameo but "in a way that feels significant". Once the logistics and creative elements were solved, Black Lightning "elevated the crossover to another level". Actor
Cress Williams said that the character enters "in a very surprising way. He's not prepared for it. So, he doesn't know these people". Because of this, there is "some conflict early on when he's trying to figure out, who are you people and why am I here?". Barry's journey through the
Speed Force in the
Arrow hour was an opportunity for the writers to "revisit a series 'greatest hits' for Oliver, key moments in terms of his relationships... it's a little bit of a 'Before you die, your life flashes before your eyes' kind of feel"; they wanted to "look backwards before Oliver's death". Oliver and
John Diggle's fight in the
Arrow season six episode "
Brothers in Arms" was originally planned for Kate's scene in the Speed Force in which she helps break up the fight. Since the writers considered season six "relatively recent", they chose "
Suicidal Tendencies" from
season three. This episode (one of Guggenheim's favorites) and the argument between Oliver and Ray helped show "how much all of the shows have evolved and relationships between the characters have evolved". The other moments in the Speed Force were based on "showing how far the characters have come". In the Speed Force, Barry encounters
Ezra Miller's
Barry Allen from the
DC Extended Universe (DCEU). Guggenheim worked with DC Comics chief creative officer
Jim Lee and
DC Entertainment film vice president Adam Schlagman on the scene to ensure that it would not conflict with their planned
The Flash film (2023), which also deals with the multiverse. Much of the dialogue between Gustin and Miller was improvised. Miller's Allen, who had not yet gone by the moniker "The Flash", is given the name by Grant's Allen in the scene. The glimpse of
Stargirl and the
Justice Society of America on Earth-2 from
Stargirl was not originally intended.
Stargirl creator
Geoff Johns said that production of that series had completed, and Pedowitz advocated its inclusion after seeing footage of the series. Johns continued, "I was incredibly excited about the opportunity for that because it just meant more people would see it. And [tonally], Stargirl and the show has always been for everybody. Anyone can watch it. So it fits in nicely to that world". The end of the crossover sees Barry converting the old
S.T.A.R. Labs hangar from the "
Invasion!" crossover into a spot where the heroes could gather, along with hinting at a possible introduction of
Gleek and the
Wonder Twins. These hints were a "fun bit" for the writers to end the crossover, since an early version of the script had the Wonder Twins making an appearance. Although early allusions to "Crisis" on
The Flash mentioned the
Reverse-Flash and
Tom Cavanagh was said to be reprising his role in the crossover, the character did not appear. Guggenheim did not want to be beholden to this, pointing out that the newspaper headline mentioning him had been created by executives no longer involved with the series or this crossover, such as
Andrew Kreisberg. The character had also already featured prominently in previous crossovers; Cavanagh nevertheless appeared in the crossover just not as Thawne. Psycho-Pirate, included in "Elseworlds", also appeared in early versions of the crossover. Guggenheim said that he was not included because "given the story we were telling, the only reason we were having him in there was because he was in the original comic". Guggenheim had tried to have
Swamp Thing appear to interact with Constantine, given their history and relationship in the comics. Although Guggenheim considered it "a really important inclusion", "for a variety of different reasons" it did not happen.
Casting LaMonica Garrett was upgraded to series regular on all the series (receiving credit in the episodes he appeared in before the crossover) to ensure that he would be available for "Crisis" and its lead-up. Casting was announced at the July 2019
San Diego Comic-Con. They included:
Tyler Hoechlin, reprising his role as
Superman, while Brandon Routh, who portrays Ray Palmer / Atom in the Arrowverse, would reprise his role as
Superman from
Superman Returns (2006) in "an older, more world-weary" version inspired by the comics'
Kingdom Come Superman; Tom Cavanagh also portraying
Pariah in addition to Nash Wells;
Burt Ward, who portrayed
Dick Grayson / Robin in the
1966 Batman television series, was cast in an undisclosed role; Amell would portray several versions of Oliver Queen;
Jon Cryer would reprise his role as Lex Luthor; and Garrett would portray the Anti-Monitor and the Monitor. Luthor was not originally intended to appear, but Cryer talked to
Warner Bros. Television president
Peter Roth about the character's prominence in the original comic to lobby for his inclusion. Shortly afterwards,
Elizabeth Tulloch was confirmed to be reprising her role as
Lois Lane. Pedowitz announced at the
Television Critics Association press tour the following month that cast members from
Black Lightning would appear (previously hinted at by Cress Williams), and Kevin Conroy would portray an elderly Bruce Wayne after voicing the character in multiple animated media. Conroy "jumped at" the opportunity to portray Wayne in live-action for the first time, saying that it would satisfy the "loyal" fanbase of
Batman: The Animated Series.
Concept art by Andy Poon displayed in
Crisis on Infinite Earths: Paragons Rising seemingly depicted
Michael Keaton filling the role of Earth-99's Bruce Wayne by reprising his role as
Bruce Wayne / Batman from the
1989-1997 Batman film series. Though Guggenheim confirmed that Keaton was never approached, a newspaper with his likeness was spotted in a leak from the Earth-89 scene. Guggenheim said that although the producers discussed with DC and
Warner Bros. the possibility of
Lynda Carter reprising her role as
Wonder Woman from the
1975 television series, they felt that it was unlikely. Guggenheim was initially hesitant in asking Ward if he was interested in returning after the idea of bringing Ward back was brought up in the crossover's
writers' room, but
The Flash line producer Joseph Patrick Finn convinced him under the logical pretext that if he didn't ask, he wouldn't get Ward in case he was interested; Ward gladly accepted to return, though the crew made sure his character wasn't identified onscreen but just hint at his identity.
Johnathon Schaech was confirmed as reprising his role as
Jonah Hex in September 2019, and John Wesley Shipp was cast in an undisclosed role shortly afterwards. Around this time, Tom Ellis was approached to appear as Lucifer Morningstar from the series
Lucifer. Ellis had "talked over the years about Lucifer being a DC character and 'Is there any chance of this happening?'", saying that "Crisis" was a good chance to explore that connection. In mid-September, it was announced that Tom Welling and Erica Durance would reprise their roles as Clark Kent and Lois Lane, respectively, from
Smallville; Durance would also reprise her Arrowverse role as
Alura Zor-El. Welling had been previously approached to reprise his role as Kent in past Arrowverse crossovers, but he felt "protective" over the fact his character didn't exist in the main reality of those shows until he was convinced to return by reading the crossover's script. Once on the set, Welling took a walk to get into "
John Schneider's spirit" due to loving the idea Clark grew up to become like his father and requested some gloves, which a
grip lent to him.
Michael Rosenbaum, who portrayed
Lex Luthor on
Smallville, said that he declined Warner Bros. and the producers' offer to reprise the role in "Crisis", as they demanded him a quick response while visiting one of his relatives with no specifics on what his reprisal would consist. The
Smallville scenes had already been shot when the producers approached Rosenbaum thanks to Amell's efforts, with Guggenheim planning to have his version of Luthor interact with Cryer's.
Alan Ritchson, who portrayed
Arthur Curry/Aquaman on
Smallville, also declined an offer to reprise his role in the crossover due to scheduling conflicts with
Titans; however, he appeared in archive footage as
Hank Hall / Hawk alongside
Curran Walters as
Jason Todd / Robin / Red Hood from
Titans.
Osric Chau was cast as
Ryan Choi later in the month, and
Ashley Scott was confirmed as reprising her role as
Helena Kyle / Huntress from
The WB series
Birds of Prey. At the beginning of October,
Audrey Marie Anderson was announced as reprising her role as
Lyla Michaels, while also taking on the
Harbinger persona from the comics; Lyla previously used "Harbinger" as her
A.R.G.U.S. codename. That month,
Stephen Lobo was cast as
Jim Corrigan. By that time, the producers approached
Nicolas Cage, who had been attached to portray Superman in
Tim Burton's planned
Superman Lives film. Guggenheim later regretted mentioning Cage and tried to refrain from hinting at other actors' involvement because he did not want fans "blowing up" their
Twitter feeds with questions about why they did not appear. At the end of November, it was reported that
Brec Bassinger would appear in the crossover as Courtney Whitmore / Stargirl before the release of her
DC Universe series later in 2020.
Robert Wuhl reprised his role as the
Gotham Globe reporter Alexander Knox from
Batman (1989) in a quick cameo appearance set in Earth-89. Wuhl, who had longed to reprise his role as Knox since his conspicuous absence in
Batman Returns (1992), readily accepted the offer to return for just a
single take of the crossover upon being approached by the executives. Ezra Miller appeared in the
Arrow episode reprising their DCEU role as Barry Allen. During development discussions for
The Flash film, it was suggested that Miller appear in the crossover, and Miller immediately agreed since they "[understand] the importance of the Flash and [the character's] role in the multiverse". Roth then spoke to Guggenheim about including Miller and Guggenheim approached Gustin to see if he would be "on-board" with Miller appearing, and Gustin "was incredibly enthusiastic" about the idea; Guggenheim called Jim Lee "instrumental in making [the cameo] happen". A number of other actors were considered and approached for cameos. There were "a whole host of reasons why" certain cameos did not occur, according to Guggenheim, as some actors were busy with other projects, some were uninterested in the crossover and others requested more money than the budget permitted, while the showrunners also had to think on those characters' screentime, their requirements in the story and the shooting schedule.
Cameron Cuffe, who starred as Seg-El on the
Syfy series
Krypton, was approached to appear as a holographic Kryptonian elder when
Argo City was evacuated. Although Cuffe said that he "wanted to make [the cameo] happen", he had committed to another project. Bringing back
Teri Hatcher as Lois Lane from
Lois & Clark: The New Adventures of Superman was similarly discussed by Guggenheim and the crew as an idea, but possibly scrapped due to Hatcher being unaffordable.
Filming Production of "Crisis on Infinite Earths" began on September 24, 2019. Filming at the farmhouse for the Kent family farm was done at the end of the month; the farmhouse had previously appeared in the "Elseworlds" crossover and on
Smallville. Production of the
Legends of Tomorrow episode began on October 4. Williams took a break from filming
Black Lightning in
Atlanta to spend a week filming his parts of the crossover in
Vancouver. His scenes were reviewed by the crew of
Black Lightning to ensure that his characterization was consistent with the series. While on Vancouver, Wuhl arrived during one afternoon to shoot his cameo seven or eight hours later. Primary filming was completed by November 8, with re-shoots and minor filming finished by December 19. When Miller's cameo was finalized, a small crew from
The Flash filmed it since the
Arrow crew had already wrapped on the series and was no longer around. Directors of the crossover included
Jesse Warn (
Supergirl), Laura Belsey (
Batwoman), David McWhirter (
The Flash),
Glen Winter (
Arrow), and
Gregory Smith (
Legends of Tomorrow). They were hired specifically for the crossover, because of their previous experience working in the Arrowverse, allowing them (and their directors of photography) to create a unified look.
Batwoman, which is normally filmed with a different
aspect ratio from the other series, used the same ratio as the other series for consistency. A number of stunt scenes, particularly in the
Supergirl and
Arrow episodes, were filmed with drones.
Music Blake Neely and collaborators Nathaniel Blume, Sherri Chung, and Daniel Chan, began work on the crossover on November 20, 2019, and the score was recorded by an orchestra on December 2.
Tony Kanal and
Stephen Perkins were featured performers on the score. Neely incorporated a number of musical
Easter eggs into the score, some of which were "cues the production has attempted to obtain the rights to for years"; cues included the
1960s Batman television-series theme, the
1989 Batman film theme, "hints" of the
Batman: The Animated Series theme, the
march theme and "Can You Read My Mind?" from the
1978 Superman film, the
1990s Flash television series theme, and the
Super Friends theme.
Effect on the Arrowverse ' character and series was merged with the other Arrowverse series to form the new Earth-Prime.|alt=A photograph of Cress Williams speaking at a convention behind a microphone Aiming to live up to the original comic book's promise that "worlds will live, worlds will die and things will never be the same" while simultaneously killing characters and introducing new ones, Guggenheim expected the Arrowverse's adaptation lived up to that. Before the crossover, Guggenheim told each of the series' showrunners that "there's an opportunity here to introduce some major
status quo shift[s]" if they desired; this was embraced by each. He described it as not "so much a burden of fixing something, it was the freedom of being able to change whatever we wanted to change". Guggenheim added that each series would be affected except for
Legends of Tomorrow, since "the crossover launches much of that season" and there would be "one rather big ramification". After the crossover, Dries wanted to "make Kate's life a little bit more integrated with all of this madness of sci-fi and aliens, and the dense mythology between" the other series, but still keep the grounding established earlier in
Batwomans season. However, this was "a little tricky because our characters aren't yet exposed to this notion of multiple universes, superheroes, powers, and stuff". An action by Barry would have ramifications for the rest of
The Flashs season. The goal of the crossover for Wallace was to "tap into that feeling that the readers in 1985 would have had, when reading that story, with the tectonic shifts in the comic world, underneath". He called the Arrowverse ending a "game-changer" which "opens up to a whole new world". By the end of the crossover, the universe was
rebooted, with all of The CW series airing at the time
Arrow,
The Flash,
Supergirl,
Legends of Tomorrow,
Batwoman and
Black Lightningexisting on the new
Earth-Prime; this was decided going into the crossover. A new multiverse was also created, with six of its Earths revealed. Although Guggenheim had wanted a single Earth-Prime, the crossover would not have been able to visit the worlds of other DC properties had that been done. A compromise was reached in which these properties were returned to different Earths in the multiverse, and the Arrowverse series were combined in a single Earth. The Arrowverse characters are oblivious to the newly formed multiverse. Guggenheim said that "there is an opportunity going forward after "Crisis" to slowly reveal all of the weird changes," with each series "get[ting] the chance to tell a piece of that story". Changes include John Diggle and Lyla Michaels' daughter, Sara, returning to the timeline after her change in
Flashpoint, and Lex Luthor having more heroic qualities. Returning baby Sara was "very important" to Guggenheim and Schwartz to accomplish in "Crisis" after they almost did so in "Elseworlds"; it was held off until this crossover because they "thought it had more punch if it was a part of the universal reboot".
Supergirl showrunners Robert Rovner and Jessica Queller came to Guggenheim with the idea to alter Lex Luthor, something they "were all really excited about". In
Batwoman, Kate's sister
Beth returns as a separate person from the season's antagonist Alice; Dries called this a "shocking resonance" for the series. The inclusion of Miller's Barry Allen from the DCEU opened up more possibilities for crossovers between the DC films and Arrowverse.
DC Films president
Walter Hamada revealed prior to "Crisis", DC had been structured in a way that the television division had to clear the use of characters with the film division. Now, the company could "really lean into this idea of [the multiverse] and acknowledge the fact there can be a Flash on TV and one in the movies, and you don't have to pick one or the other, and they both exist in this multiverse". Berlanti agreed, feeling that "moving forward, there's more opportunity to do more things like this". ==Marketing==