WARNING: The following feature contains SPOILERS for the 100th episode of The Flash.
The Flash reveals that Nora West-Allen is working with the Eobard Thawne, aka the Reverse-Flash, in the future. Previous episodes had hinted that the daughter of Barry Allen and Iris West-Allen from the year 2049 had several secrets she was hiding from her parents, as well as Team Flash. This was confirmed in the final scene of The Flash season 5 midseason finale, "What's Past Is Prologue," as Nora traveled forward in time to report to her master in-person.
The plot of The Flash's 100th episode saw the team trying to think of a way to counter the powers of Cicada - a serial-killer empowered by a meta-tech dagger which allows him to neutralize the powers of metahumans who gained their powers from exposure to dark matter. Nora hit upon the idea of building a piece of meta-tech of their own that could be used to neutralize Cicada's dagger. suggesting a supercharged magnet that could hold the dagger in place and cancel-out its power-dampening abilities. This led to a scavenger hunt across time, as Barry and Nora returned to the scenes of several of Barry's greatest battles to retrieve the materials they needed, including a piece of Savitar's armor and the device Zoom used to steal Barry Allen' speed.
While Nora was otherwise preoccupied, Sherloque Wells made a study of Nora's private journal and determined that it was written in some strange code he didn't recognize, but Cisco identified as the strange code Barry briefly wrote in after his return from the Speed Force at the start of The Flash season 4. When Sherloque questioned Nora about the language, she claimed it was a special time-traveler language she invented that allowed her to keep track of things no matter what changes may have occurred in the timeline.
Later in the episode, we see Nora using the Gideon AI in the STAR Labs Time Vault to transmit her latest notes to someone else. When Gideon asks if Nora would like to attach a message, Nora says she'll deliver the message in person and runs forward in time to Iron Heights Prison in the year 2049. We then see her approach the cell of a familiar looking figure - Eobard Thawne, the Reverse-Flash, who is still in the guise of Harrison Wells.
Even devout fans of the Arrowverse may be wondering just how the Reverse-Flash can still be alive at any point in time. Barry Allen's greatest enemy, who has changed the timeline more than once in his efforts to kill Barry Allen or stop him from becoming the Flash, was seemingly erased from existence in The Flash season 1 finale, after his ancestor, Eddie Thawne, killed himself in order to stop the Reverse-Flash from ever being born. A remnant of the Reverse Flash, desperate to outrun the Time Wraiths that sought to eliminate him for being part of a redundant timeline, formed the Legion of Doom in Legends of Tomorrow season 2, in a bid to recover the Spear of Destiny - an artifact that would give Thawne the power to rewrite reality so he could exist despite the paradox that should have killed him. But with Gideon saying that Nora is a "future recruit of the rebooted Legion of..." something before being cut off, considering she's working with or for Thawne, it's theoretically possible that Nora is a member of the Arrowverse's second Legion of Doom (if there ends up being one).
This final scene of The Flash season 5's midseason finale raises many questions which have no easy answers, even beyond how Thawne is still alive in any form. Doubtlessly, the second half of season 5 of The Flash will reveal this as well as why Thawne is in a prison in 2049 and why he's still wearing Harrison Wells' face. There will also, hopefully, be answers as to just why Nora West-Allen is helping her father's greatest enemy to gather information from the past. Perhaps the biggest and most frightening question of all is just what the Reverse-Flash is planning to do with this information once he has gathered everything he needs.