WARNING: This article contains major spoilers for "Legacy," the Season 5 finale of The Flash.
The Flash may have just set up Reverse-Flash's demise. Throughout Season 5, Eobard Thawne mentored Nora West-Allen in the year 2049 in an effort to manipulate her into destroying Cicada's dagger, which prevented him from using his powers and escaping prison. Despite his ulterior motives, he grew attached to Nora, and that may be his undoing.
Because Thawne's plan worked, and the dagger was destroyed, a new timeline began to set in. As a result, Nora began to disappear; by defeating Cicada early, Team Flash inadvertently created a timeline in which she was either born at another point or else not at all. When they realized what was happening, Thawne apologized to Nora, using his pet name for her: "I'm sorry, Little Runner, for trading you in like this, but I had to be sure that I could go free."
Thawne even ensured she could be saved -- albeit while allowing himself to escape. "You can still save her, Barry," he told The Flash. "The Negative Speed Force. She has to go there. It's the only place immune to timeline changes, but you have to go now." When Barry hesitated, Thawne got agitated, saying, "You have to go, Barry! Or we lose her forever. That's right. I've grown fond of her too. In many ways, she's shown me what it's like to have a daughter."
Ultimately, Nora decided to allow the new timeline to take her. She refused to let the Negative Speed Force twist her as it had Thawne. As such, she permitted herself to crumble away, and disappeared from the timeline, leaving only her memory. Barry and his wife Iris were, of course, devastated, but Thawne was long gone at that point.
Nora's disappearance wasn't the only change to the timeline, however. Thanks to his meddling, the timing of "Crisis on Infinite Earths" bumped up to 2019 instead of 2024. This was confirmed in the stinger of the episode, when the text of of the infamous "Flash Missing Vanishes in Crisis" article changed slightly. For one, the date reflected the new 2019 timestamp; for another, it confirmed the involvement of both Batwoman and the Legends of Tomorrow, and dropped Hawkgirl entirely. What is perhaps the most significant information was left unaltered.
The text on the front page of the paper, as it appeared in "Legacy," can be read in full below:
After an extreme battle with the Reverse-Flash, our city's very own Scarlet Speedster disappeared in an explosion of light. The cause of the fight is currently unknown. According to witnesses, The Flash, with help from Star City's Green Arrow, White Canary, Supergirl, Batwoman and a slew of other legendary heroes, began fighting the Reverse-Flash around midnight last night. The sky took on a deep crimson color as the ensuing battle created the most destruction this city has seen since The Flash first arrived to Central City.
Several trucks were caught in the fray, spilling their contents into the street. Power outages swept nearly twenty city blocks, between 16th Street and Adams Avenue. Five of those blocks still remain without power. All of the buildings in the area were evacuated by the CCPD, with additional help from The Atom.
According to reports, as The Flash and Reverse-Flash battled with each other between two overturned tanker trucks, the lightning emanating from the speedsters threatened to ignite spilled oil leaking from one of the trucks.
The smoke from the truck engine made it difficult to see, but it appeared at one point they were having a very heated argument before continuing the fight. Then, suddenly, The Flash sped after Reverse-Flash, and the two vanished, leaving Green Arrow and the others behind.
An eyewitness who watched the battle from her apartment building before being evacuated said, "It was hard to see, but The Flash and Reverse-Flash were zipping up and down Monroe Avenue and then there was a whole bunch of lightning, and then nothing. It was weird."
Other witnesses described a familiar phenomenon: a blinding light followed by darkness. Then the sky returned to black. And as the street quieted, it took only a moment to realize The Flash and Reverse-Flash were gone without a trace.
Central City Police Captain, Joe West, held a press conference in the wee hours of the morning, stating, "We don't have very many details right now. What we do know is tonight Central City's greatest protector vanished in a flash. We can only hope he returns just as quickly." A sentiment shared by all who call this great city home.
The Flash and Reverse-Flash were always destined to have a blowout during "Crisis on Infinite Earths," but "Legacy" may have clued us in on what it was about. Naturally, Barry is furious about his daughter's demise. When he comes to learn of it, Thawne likely will have regrets as well, seeing as he began to think of Nora as a daughter. After all, it is his fault, since he bumped "Crisis" up so much that he ultimately prevented Nora's birth.
In fact, Thawne may regret what happened to Nora so much that he will sacrifice himself to save Nora. As in the comics, Barry was originally destined to sacrifice himself to save the multiverse in "Crisis." Although The Flash didn't come out and say he died in the event, he never returned after the moment mentioned in this article; Nora confirmed as much when she showed Barry another article revealing he was still missing 25 years later. This suggests he dies, which is why he never returns. However, if Thawne feels as strongly as he claims about Nora, he may end up taking Barry's place.
In the article above, both Flash and Reverse-Flash disappear. Notably, Flash is in pursuit of Thawne when this happens. If they disappear together, they are likely heading to the same place, which makes this swap feasible. What's more, the result of this chase is left as a mystery, indicating that "Crisis" will explore what happens on the other side.
By taking Barry's place, Thawne would almost ensure Nora's existence. If he died in Barry's stead, Barry would be able to return home to Iris and start a family, which could lead to the existence of Nora and perhaps other children. This would also allow The Flash as a series to continue. If Thawne was genuine about his paternal feeling for Nora, the decision to save Barry -- and the multiverse at large -- would make sense for the character, as established by Season 5.
As it stands, Oliver Queen is destined to die in "Crisis," as he and many other notable characters did in the comic book event of the same name. It wouldn't be out of line for a major Flash character to go as well. Plus, a multiverse-saving sacrifice would be a nice endcap for a character like Reverse-Flash, whose twisted affection for Team Flash has persevered despite his deadly intentions.
Returning this fall on The CW, The Flash stars Grant Gustin, Candice Patton, Carlos Valdes, Danielle Panabaker, Tom Cavanaugh, Jesse L. Martin, Danielle Nicolet and Hartley Sawyer.