SPOILER WARNING: This article contains major spoilers for "Legacy," the Season 5 finale of The Flash.
Following Arrow's monumental setup for "Crisis on Infinite Earths," this year's four-show Arrowverse crossover, The Flash dropped a few hints about the major event. In "Legacy," the Season 5 finale, Reverse-Flash himself Eobard Thawne teased his involvement. He also confirmed that Team Flash created a new timeline, which moved "Crisis" up from 2024 to 2019.
Team Flash accidentally created a new timeline when they destroyed Cicada's dagger. When Barry and Iris' daughter Nora began to disappear, Thawne confirmed this was a consequence of their actions. "A new timeline is setting in," he explained. "A new timeline that you created when you destroyed the dagger. I'm sorry, Little Runner, for trading you in like this, but I had to be sure that I could go free."
Reverse-Flash gave Barry a choice: save Nora by bringing her into the Negative Speed Force or contain him. Barry, of course, chose to try and save Nora. Enraged, he released Thawne, who offered him a devious smile in response. "See you in our next Crisis," he said, and took off. This suggests he will be involved in "Crisis on Infinite Earths" when it arrives this fall.
The episode's stinger also heralded the multiverse-ending event. After the title card, the show cut to the Time Vault, where the A.I. Gideon suddenly woke herself up. "Incoming time flux," she announced, lighting up. She projected an image of the infamous "Flash Missing Vanishes in Crisis" newspaper, which began to change. The date -- April 24, 2024 -- began to tick backwards and stopped at 2019, which aligns the in-universe timeline with The CW's Arrowverse crossover plans.
Last night's Arrow Season 7 finale also provided more clues about "Crisis on Infinite Earths." The Monitor came to collect on Oliver's side of the bargain he struck in "Elseworlds": that he fulfill "whatever the multiverse requires to survive the Crisis that is looming, when it is time." The Monitor also confirmed that Oliver would die during the "Crisis," saying, "I cannot prevent his passing, but he can prevent the deaths of countless more." Oliver made good on his promise and left with the Monitor just as the episode concluded.
Returning this fall on The CW, The Flash stars Grant Gustin, Candice Patton, Carlos Valdes, Danielle Panabaker, Tom Cavanaugh, Jesse L. Martin, Danielle Nicolet, Hartley Sawyer and Jessica Parker Kennedy.