Warning: This article contains Crisis on Infinite Earths comic SPOILERS.
According to the Monitor (LaMonica Garrett), Barry Allen will meet his end during the upcoming Arrowverse crossover, Crisis on Infinite Earths - and there's a reason for that. It would seem that The Flash's Barry Allen (Grant Gustin) and Arrow's Oliver Queen (Stephen Amell), who are the protagonists of the first two Arrowverse shows, are both destined to meet the same fate in the event. But Barry's fate, in particular, is directly related to the comics. Just like his TV counterpart, the comic book version of Barry Allen had to die so that the Crisis could be averted.
Barry Allen's death in the Crisis on Infinite Earths has been foreshadowed since the pilot episode of The Flash when Barry saw the newspaper from the future which teased his demise. Originally, Barry's death was supposed to occur in 2024, but the events of the season 5 finale seemingly moved the Crisis to the immediate future, which means that Barry doesn't have much time before he "vanishes". Up until this point, The Flash has been ambiguous about what Crisis on Infinite Earths has in store for Barry. In the Flash season 6 premiere, the Monitor assures Barry that in order for the multiverse to be saved, he must die. In last week's episode, Barry finally comes to terms with the fact that his death is a necessary sacrifice.
After a 30-year journey in DC Comics that began in 1956, Barry's story was given an ending in Crisis on Infinite Earths. Barry Allen was living peacefully in the 30th century when he got swept into a conflict with the villain of Crisis, the Anti-Monitor. The Anti-Monitor was going to wipe out the multiverse with his antimatter cannon, and the only person capable of stopping him was the Flash. In order to prevent the antimatter from destroying the entire universe, Barry ran faster than ever before to counteract the antimatter and was thus swallowed into the Speed Force.
Barry Allen's actions in Crisis on Infinite Earths saved the multiverse but also cost him his life as he entered the Speed Force forever. Barry remained dead until the character was finally revived in 2008; he was replaced by Kid Flash in the years since he disappeared after Crisis. Barry's death is perhaps the most noteworthy and most meaningful sacrifice in DC Comics history, even though it didn't necessarily end the Crisis as a whole.
Barry stopped the Anti-Monitor but never defeated him. What Barry did do was make it possible for the heroes to eventually beat the Anti-Monitor. If the Arrowverse takes Barry in the same direction, that could explain why both Barry and Oliver need to die. Barry may have to die first in order to set their victory in motion, and Oliver's sacrifice will come later in the crossover. But, of course, the Arrowverse doesn't always adapt comic stories exactly as they were told on the page, so Barry may ultimately survive in the end.