Heroes Join Forces

"Heroes Join Forces" is the second Arrowverse crossover event that features episodes of the television series The Flash and Arrow on The CW. The event began on December 1, 2015, with The Flash episode "Legends of Today" and concluded the following day with the Arrow episode "Legends of Yesterday". The episodes feature characters from Legends of Tomorrow, which was being developed as a spin-off. The crossover sees Barry Allen / Flash and Oliver Queen / Arrow team up to take on Vandal Savage, who is looking for Kendra Saunders and Carter Hall, the reincarnations of Hawkgirl and Hawkman.

A crossover between the two series was announced in January 2015 by The CW President Mark Pedowitz, who stated the network's intention to have an Arrowverse crossover every season following the success of the previous year's crossover "Flash vs. Arrow". By September, the scripts for each episode had been written, with filming occurring shortly before the end of the month and into October 2015. The crossover sees every main cast member of each series who appear at least in their own series, with additional actors and characters also reprising their Arrowverse roles.

The episodes received generally positive reviews; however, the Legends setup received mixed responses. Both series experienced season high viewership despite the ratings being down from the previous year's crossover. A subsequent crossover occurred the following year, titled "Invasion!".

Vandal Savage arrives in Central City looking to kill Kendra Saunders. After he attacks Kendra and Cisco Ramon, Barry Allen takes Kendra to Star City and enlists the help of Oliver Queen and his team to protect her. The team is visited by Malcolm Merlyn, who informs them that Savage is an immortal. Later, Kendra is kidnapped by a mysterious winged man, but Barry and Oliver rescue her and capture him. He introduces himself as Carter Hall, and tells them he and Kendra are soulmates who have been connected for millennia. They are destined to die, be reborn, and find each other in each lifetime. Carter also reveals that Savage has killed the pair in all of their previous lives, each time growing stronger, while Savage acquires the Staff of Horus, a deadly mystical weapon. Kendra unlocks her abilities, with Cisco naming her Hawkgirl, and the team decides to regroup in Central City. While there, Oliver witnesses his ex-girlfriend Samantha Clayton with her son William, and realizes the child is likely his. Meanwhile, Caitlin Snow and Harry Wells create a serum capable of temporarily increasing Barry's speed so that he can defeat Zoom. Jay Garrick initially refuses to test the serum, but is later forced to do so in order to save Wells' life when he is shot by Patty Spivot, who mistakes him for Eobard Thawne. Jay advises against using the serum on Barry.

Malcolm orchestrates a meeting between Savage, Barry, and Oliver. Savage demands they turn over Kendra and Carter, threatening to destroy both Central City and Star City with the Staff of Horus. Without Felicity Smoak's knowledge, Oliver has Barry perform a paternity test on a strand of William's hair, which confirms that he is William's father. Confronting Samantha, she agrees to let him see William on condition that he tell no one, including William himself, of the boy's parentage; Felicity learns of Oliver's paternity from Barry. Frustrated and feeling betrayed that Oliver is still willing to keep secrets from her, she ends their relationship.

Barry and Oliver devise a plan to deliver Kendra and Carter as a ruse to get close enough to Savage to destroy the staff. The plan goes horribly wrong; Kendra's powers fail and Savage quickly gains the upper hand, killing both her and Carter before using the staff to destroy Central City. Barry escapes and runs back in time to the point of the original negotiation. Barry informs Oliver of his time travel and the mistakes that led to their defeat. They change their approach to the plan and Barry is able to steal the staff. He and Oliver use it on Savage, turning him into ashes. Kendra and Carter decide to use their powers to help others in another city. Cisco gives Kendra a tracking device. Oliver agrees to Samantha's conditions for him to see William. Even though Barry encourages Oliver to tell Felicity about William, who she never learned about due to Barry's time traveling, he decides not to reveal the secret to her. Later Merlyn collects Savage's ashes while saying that Savage owes him.

In January 2015, The CW President Mark Pedowitz stated the network's intention to have an Arrowverse crossover every season, after the previous year's two-part event with the eighth episodes of the third season of Arrow and the first season of The Flash, titled "Flash vs. Arrow". In July 2015, Andrew Kreisberg, creator and executive producer on both series, confirmed that the eighth episodes of the fourth season of Arrow and the second season of The Flash would be a two-part crossover. The next month, Kreisberg revealed that the crossover would help lay the groundwork for the new team-up, spin-off series Legends of Tomorrow. Kreisberg said, "Both Arrow and Flash are helping to set those things up, in both big ways and small ways, which is lovely... There's going to be a lot happening leading up to the crossover, which is serving as a [sort-of] pilot for Legends."

Kreisberg described the crossover between both shows as "a superhero a-go-go!", saying: "It's insane to have them all together." Unlike the 2014 crossover, which was two separate self-contained adventures that involved each show, Marc Guggenheim, executive producer on Arrow, said: "This [year's] is a crazy, full-on 'To Be Continued' crossover,... it really feels like a 2-hour movie [airing] over two nights." Kreisberg and Guggenheim also reassured fans that both episodes would not focus solely on setting up the spin-off series, and that the crossover is still Oliver Queen and Barry Allen having their own adventure. "These [episodes] have important stuff going on for the narratives of both The Flash and Arrow," said Kriesberg, while Guggenheim added: "The final moments of The Flash have a pretty big jaw-dropper and we pick up those threads in Arrow. Then we do some things with the characters from Arrow... that are pretty epic and major."