SPOILER WARNING: This article contains major spoilers for "What's Past is Prologue," the 100th episode of The Flash.
Reverse-Flash just invaded Barry Allen's life in a brand new way. "What's Past is Prologue," the 100th episode of The Flash, revealed Nora West-Allen didn't decide to go to the past on her own. She has been working with Eobard Thawne all along.
The milestone episode saw Barry and Nora travel to the past in order to pick up a few rare items they needed to create a magnet strong enough to negate Cicada's power-dampening dagger. Along the way, they bumped into Thawne at a time before his Season 1 defeat. Nora was incredibly intrigued -- and a little too trusting -- of Thawne. Though Barry was hesitant to answer Thawne's searching questions, Nora supplied them readily, much to Barry's dismay.
When they got a moment alone, Barry asked if she knew what Thawne had done. Nora regurgitated some of the information she learned from the Flash Museum, not really grasping the magnitude of Thawne's impact on Barry's life. This forced Barry to reveal how Thawne had killed his mother in an effort to make him so broken that he never became the Flash. Nora was absolutely floored by this revelation and, while her hurt seemed to be on her dad's behalf, there was much more to the story.
At the end of the episode, Nora scanned her latest journal entry with Gideon's help. She began to leave a message for someone on the other end, but stopped and decided to deliver it herself. With that, she headed back to the year 2049 and paid a visit to Iron Heights. When she stopped, she stood outside a cell where a man read her journal entry with his back to her. He turned, revealing an older version of Eobard Thawne, still stuck with Harrison Wells' face.
"We have to talk," Nora spat. Locking eyes with her, he replied, "Yes, Nora. Yes, we do."
The last time we saw Reverse-Flash, he had teamed up with Dark Archer and Overlord for the events of "Crisis on Earth-X." In the crossover's climactic battle, Barry finally got the upper-hand on Thawne, but made the surprising decision to let him go. Thawne sped off with an ominous warning: "I wonder what face I'll be wearing next time we meet, Flash." That is the last time he was seen in the Arrowverse -- until now.