WARNING: The following feature contains SPOILERS for The Flash episode 'What's Past Is Prologue'.
The mid-season finale of The Flash may have hinted at the return of the Legion of Doom with its twist ending. 'What's Past Is Prologue', which was also the 100th episode of The Flash, contained a number of surprises. Yet none was quite so shocking as the subtle suggestion that the Arrowverse's greatest team of villains may have been reborn (or will be reborn shortly) with Nora West-Allen as part of its membership.
Introduced as the main menace of Legends of Tomorrow's second season, the Legion of Doom was comprised of various villains who had already met defeat at the hands of the heroes of the Arrowverse. Led by Eobard Thawne - the Reverse Flash - the Legion of Doom sought the Spear of Destiny - a magical artifact that gave its wielder the power to rewrite reality. The Legion of Doom was briefly successful, creating an alternate timeline called Doomworld where their darkest desires were made reality. Thankfully, the efforts of Mick "Heatwave" Rory and a rejuvenated team of Legends restored the timeline and seemingly erased Eobard Thawne from existence once and for all.
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It seems you can't keep a good villain down, however, and Thawne was revealed to be alive, in prison, and once again in the guise of Harrison Wells in the year 2049. It was also revealed that Nora West-Allen had been sending him information about the current state of Barry Allen's life in 2018 using the Gideon A.I. inside the STAR Labs Time Vault. The idea that Nora was in Thawne's employ was surprising enough, yet another encounter with Gideon earlier in the episode may suggest an even darker side to Nora's nature.
'What's Past Is Prologue' saw Barry Allen and Nora traveling back in time to the night of STAR Labs' particle accelerator explosion, in order to create a piece of meta-tech that could counter the dagger utilized by the metahuman serial killer Cicada . While they were there, Barry and Nora spoke with the Gideon A.I. in the Time Vault and Gideon expressed surprise to see her creator, Barry Allen, at this point in time. She also revealed that she recognized Nora West-Allen and began to describe her history according to the historical record, saying that Nora was "also known as XS" and was the "fifth recruit in the rebooted Legion of - " before Barry cut her off.
Fans of the Legion of Superheroes probably saw this as an Easter Egg nod to the comic-book origins of XS, which was the codename of Barry Allen's granddaughter, Jenni Ognats, when she was introduced in Legionnaires #0 in 1994. Jenni was one of the first recruits to what became known as '"The Reboot Legion" after the original history of the Legion of Superheroes was rebooted, as part of the Zero Hour event. However, with Nora having been revealed to be working for Eobard Thawne and Gideon having been shown to have no ability to hide information from those she is explicitly programmed to obey, there is a chance that Gideon might have been about to say "Legion of Doom" rather than "Legion of Superheroes" before Barry cut her off.
Another clue as to Nora's allegiances may lie in the fifth season premiere of The Flash. It was here that Nora made a reference to the hero Lightning Lad - another member of the Legion of Superheroes. Many took this as a sign that the Arrowverse version of XS was also a part of the Legion of Superheroes. What is interesting is that in the Reboot Legion timeline, the young man originally called Lighting Lad adopted the codename Live Wire instead. Could this incongruity indicate that Nora was not really part of the Legion of Superheroes and is merely a pawn whom Eobard Thawne trained to impersonate Barry Allen's daughter? It's certainly a possibility to consider when The Flash returns with new episodes in 2019.