WARNING: The following contains spoilers for The Flash Season 7, Episode 18, "Heart of the Matter, Part 2," which aired Tuesday on The CW.
Barry Allen and Team Flash have been extraordinarily busy in The Flash Season 7, defending Central City from rogue Forces and an army of Godspeed drones traveling back from the year 2049. Through it all, Barry and Iris have reaffirmed their marriage, the team has lost several key members over the course of the year and they've welcomed Barry and Iris' superhero speedster children from the future to help them save the day.
And while Team Flash was ultimately successful in defeating Godspeed and restoring peace to Central City, the final battle came with the startling reveal that Barry's greatest nemesis has officially returned: Eobard Thawne, the Reverse-Flash.
As the Flash's most hated foe, Thawne had traveled back in time and murdered Barry's mother Nora Allen when Barry was just a young boy, continuing their vendetta when Barry eventually claimed his superhero destiny. Since then, the Reverse-Flash has died and returned from the grave several times, from Eddie Thawne killing himself to prevent his distant descendant from ever being born to a time remnant of Thawne being erased from reality by the Black Flash during Legends of Tomorrow. Following "Crisis on Infinite Earths," Thawne's consciousness lived on in Harrison Wells, attempting to take control of his body, before he seemingly died with Wells when he gave his life to resurrect the Speed Force.
With Barry running the risk of being overwhelmed by the newly upgraded Godspeed, the Scarlet Speedster surprisingly turns to Thawne for help, with the Reverse-Flash agreeing to temporarily set aside differences against a common enemy. While Godspeed puts up a valiant fight, two speedsters against one, along with Thawne's more brutal tactics, leads to the white-suited supervillain being gravely wounded. Thawne then turns his attention back towards killing Barry only to discover that the reborn Speed Force has made the Flash far faster than him. Shocked at this development, the Reverse-Flash escapes and vows to find a way to grow even faster and finally achieve his vengeance against Barry.
Back at S.T.A.R. Labs, Barry and Iris' daughter Nora is surprised at how the Reverse-Flash was able to come back, indicating that her parents did something to disrupt the timeline that she was more familiar with. Nora and Iris reveal that when they restarted the Speed Force in the beginning of Season 7, the event also led to the creation of the Negative Speed Force that empowers the Reverse-Flash. Barry and Iris reason that the Negative Speed Force serves to bring balance to the Speed Force and prevent it from raging out of control. With the Negative Speed Force restored, so too was Eobard Thawne's existence.
One way or another, a final showdown between the Flash and Eobard Thawne was always in the cards; more a matter of when and how than if. Barry has his speed back and is faster than ever before, but at the price of the Reverse-Flash back retuning, deadly as ever. With the timeline subtly changed by Barry and Iris' actions, there is no telling where or when the Reverse-Flash may go next in order to boost his own speed and carry out his revenge. And with the Reverse-Flash on the loose, the supervillain is always at his most dangerous when the heroes never see him coming for them.
Season 7 of The Flash is now complete and is available to stream in full on The CW app.