WARNING: The following article contains SPOILERS for the 100th episode of The Flash.
The twist ending of the mid-season finale of The Flash was apparently spoiled several weeks earlier by an Easter egg referencing Batman Beyond. This twist seemingly confirmed a certain fan theory regarding the true loyalties of Nora West-Allen, and suggests dark times ahead for Team Flash and Barry Allen.
First seen during the wedding of Barry Allen and Iris West during the Crisis on Earth-X crossover event, Nora West-Allen was officially introduced in the season 4 finale of The Flash. Claiming to be both Barry and Iris's daughter from the future and trapped in the year 2018, Nora was quickly revealed to be concealing the truth, and confessed that she had lied about the anti-tachyons that left her unable to time-travel. Nora claimed that she wanted to get to know the father she only knew as a legend, as Barry Allen disappeared while saving the world when she was a toddler. A DNA test seemed to confirm that she was Barry and Iris' daughter, but nobody on Team Flash thought to ask if she had faked that along with the presence of the anti-tachyons they had detected before.
Nora fought crime alongside her father throughout the first half of The Flash's fifth season, dropping occasional hints as to her life in 2049 and bits of futuristic slang taken from various alternate futures depicted in the classic DC Comics books, such as the Legion of Superheroes. One of these words - "schway" - was unique to the setting of the animated series Batman Beyond. Ignoring the possibility of this being a mere Easter egg meant to amuse fans of the comics and cartoons, one theorist suggested that Nora's use of the word implied a connection to Eobard Thawne, the Reverse Flash.
As utterly insane as the reasoning behind the theory was, the point was shockingly affirmed by "What's Past Is Prologue" - an episode that served as The Flash's 100th episode and mid-season 5 finale. The end of the episode not only confirmed that Eobard Thawne was somehow alive and once again wearing the face of Dr. Harrison Wells in the year 2049, but aslo revealed that he was in prison and that Nora West-Allen had been sending him information about Barry Allen's life in 2018. The information was written in a special time language that can reportedly keep track of data in spite of any changes to the timeline caused by a speedster changing historical events.
It remains to be seen what Thawne is plotting and what role Nora West-Allen (if that is her real name) will have to play in the Reverse Flash's latest scheme to ruin Barry Allen's life. Of course turning Barry Allen's daughter to the path of darkness would be a suitably evil scheme for Thawne's twisted mind. So would making Barry fall in love with a young woman he thought was his daughter, only to reveal that he'd been played for a fool for several months. Whatever the case, the second half of the fifth season of The Flash will be something to watch avidly in 2019.