The greatest villain Flash has ever faced off against has only ever appeared once in DC's vast history. Barry Allen's futuristic nemesis got a horrifying upgrade that made him one of the deadliest speedsters around.
Despite being the Fastest Man Alive, the Flash has a number of villains who continuously challenge him. But for all the Rogues or Gorilla Grodds that have tried to defeat the speedster, none have been as big a threat to him as the Reverse-Flash. Hailing from the 25th century and hating Barry Allen more than anything, Eobard Thawne gave up everything to become the Reverse-Flash and make the Scarlet Speed's life a living hell. His hatred for Barry is so powerful that Thawne learned everything he could about manipulating the timeline just so he could ruin Flash's life. Thawne's coup de grâce, of course, was killing Barry's mother and framing his father, leaving Allen to grow up an orphan.
Reverse-Flash's Black Lantern Form Is a Nightmare
However, a brief brush with an evil Power Ring took Reverse-Flash’s game to a new level of villainy. In Blackest Night: The Flash #1 by Geoff Johns and Scott Kolins, the DC Universe is under attack by the Nekron, the Lord of the Unliving and his Black Lantern Corps. To aid in his mission to destroy all life, he’s sent out rings across the universe to reanimate heroes and villains and add them to his ranks. One such villain is Eobard Thawne, who comes back to life and seeks out the Flash in revenge for accidentally snapping the Reverse-Flash’s neck and killing him. Knowing that's what happened to Thawne is spreading all across the Earth, Barry chooses to run instead of engaging in a conflict with his zombified archenemy.
The undead Lanterns are actually simulations created by their Power Rings, but they possess all the same powers the Black Lanterns had in life. Not only that, they also possess the abilities Power Rings give Green Lantern, such as flight and construct generation. So while this version of the Reverse-Flash isn’t technically Thawne, he’s just as powerful and capable of wielding his speed as the villain was. In fact, with the added bonus of the Power Ring, he’s possibly the most powerful version of Barry’s nemesis there’s ever been.
Reverse-Flash with a Power Ring is A God-Level Danger
No matter how much Barry Allen understands the Speed Force, he’s always going to be repeatedly flummoxed by the one person who also knows it intimately. Eobard Thawne’s mastery of time travel has made him one of the most dangerous people in the entire DC Universe. But throw in the unchecked power of a Black Lantern Power Ring and he’s suddenly a god-level threat. A monster that can go to any point in time, take down nearly anyone with functional immortality? Barry is just lucky that the Reverse-Flash’s membership to the Black Lantern Corps was temporary, because he could have been Flash’s worst nightmare.