Spoilers ahead for the fifth season finale of The Flash.
The Arrow-verse is adapting the iconic DC Comics arc known as "Crisis on Infinite Earths" for its 2019-2020 crossover event, and the Season 5 finale of The Flash revealed a key detail. Flash viewers have been predicting a "Crisis on Infinite Earths" storyline from almost the very beginning, when the future newspaper announced that The Flash would disappear in a "crisis."
Confirmation that "Crisis on Infinite Earths" is coming in the fall proved that the Arrow-verse really was going for it, but there were more questions than answers. The biggest, arguably, was about the timing.
The newspaper originally stated that The Flash would go missing in a crisis that goes down in 2024, but the Arrow-verse is still several years shy of that mark. Would "Crisis on Infinite Earths" take the Arrow-verse into the future for the crossover event, or would something happen to make the "crisis" happen early?
The Flash's Season 5 finale provided the answer: the "crisis" will happen in 2019 rather than 2024. What caused such a monumental change in the timeline? Sadly, it was seemingly the death of adult Nora West-Allen, which set off a "time flux."
The timeline was messed up more than ever in the Season 5 finale when the good guys destroyed Cicada's dagger in the present, freeing Reverse-Flash. He raced back to 2019 with Barry and Nora hot on his trail, and the rest of Team Flash joined in to help defeat him. Unfortunately, Reverse-Flash had an ace up his sleeve, as he often does.
The changes in the timeline meant that Nora was going to fade away and die unless she embraced the Negative Speed Force. Panicking, Barry and Iris tried to make their daughter utilize the Negative Speed Force, but she refused, saying that if she used it to save her life then it would be part of her forever, and she wouldn't let that happen. Nora died, Reverse-Flash escaped, and it was an all-around bummer of a twist.
Reverse-Flash did drop a comment to Barry that they would meet at the next crisis, so the odds are pretty good that he'll appear in "Crisis on Infinite Earths." That was always likely, but getting a solid hint in the Season 5 finale gives fans something to speculate about during the months of hiatus.
So, thanks to The Flash's fifth season finale, we know that the "crisis" happens in 2019 and Reverse-Flash is almost certainly going to have a part to play. This news comes shortly after the Arrow Season 7 finale dropped some "Crisis on Infinite Earths" clues of its own.
Spoilers ahead for the Arrow Season 7 finale. In the Arrow Season 7 finale that said goodbye to Felicity Smoak, The Monitor returned to collect on Oliver's promise to assist him in preventing Barry and Kara's deaths. He also seemed to confirm Oliver's death, which many fans were predicting from the end of the "Elseworlds" crossover, but a twist at the end of the hour indicated that Oliver survives... just somewhere far from Earth-1. End Arrow spoilers.