Why the Arrowverse made The Flash’s Eobard Thawne (aka, Reverse-Flash) its most important villain. Reverse-Flash, the arch-nemesis of Barry Allen, has had a resurgence in The Flash season 8. He's been played by two actors — Tom Cavanagh and Matt Letscher — over the course of his time in the Arrowverse and his continued presence has made him a crucial element to the fabric of the shared universe for numerous reasons.
Born in the 23rd century, Eobard Thawne despised The Flash after the Scarlet Speedster appeared in his time and saved a crowd of people just as Thawne was about to dazzle them with his own newly acquired speed powers. Reverse-Flash grew so angry and bitter towards Barry for (unintentionally) taking his moment that he ran back in time and killed his mother, vowing to never stop making The Flash’s life miserable. Since then, the nefarious speedster has not only schemed and plotted against The Flash, but he’s led the Legion of Doom — which included Damien Darhk and Malcolm Merlyn — in Legends of Tomorrow season 2.
Theoretically, any other Arrowverse villain could have led the Legion of Doom, a group of supervillains usually helmed by Lex Luthor in the comics. However, Reverse-Flash was the ideal candidate because of his relationship to the Scarlet Speedster. The Flash has been (and still is) The CW’s most popular Arrowverse series. While its ratings have steadily been in decline over the years (as is what typically happens with most shows as they age), The Flash remains the flagship of the Arrowverse. To that end, it makes sense to take the most popular show’s biggest villain and have him spearheading the Legion of Doom on another Arrowverse show. Malcolm Merlyn and Damien Darhk joined Thawne, but they had a lot less power in the group dynamic than he did. Most fans already knew who Reverse-Flash was thanks to his time on The Flash and a previous mention of the villain on Arrow, so it seemed inevitable he would go on to lead his own team.
Dropping him into Legends of Tomorrow season 2 was an extension of his story on The Flash and simultaneously tied into the Legends’ time traveling shenanigans. The popularity of The Flash also led to Reverse-Flash appearing as one of the villains in the Arrowverse crossover “Crisis on Earth-X,” where he joined the evil doppelgängers of Green Arrow and Supergirl. His rivalry with Barry, as well as the fight to save himself from being erased from existence, is a journey that has spanned the Arrowverse. No other villain has gotten the same treatment and it’s a testament to The Flash being so well-known, which boosts Thawne’s notoriety and makes him a prime candidate to appear elsewhere (and so seamlessly).
The Arrowverse has had its fair share of villains since 2012. On Arrow, Slade Wilson and Prometheus terrorized the Green Arrow and his team for a long time. Meanwhile, Supergirl saw the likes of Lex Luthor, Reign, and Nyxly run rampant throughout National City. Legends of Tomorrow and Batwoman have similarly battled a plethora of nemeses. However, it’s Reverse-Flash who managed to work his way into the mix more deeply, interacting with the Arrowverse’s villains and superheroes the most out of all the major antagonists — including Lex Luthor, who is arguably one of the most familiar faces from DC Comics. With The Flash maintaining its top spot since season 1, it’s no wonder the speedster’s biggest villain has gotten so much time to shine throughout the Arrowverse.