Many spoilers ahead for Episode 21 of The Flash Season 5, called "The Girl With The Red Lightning."
The fifth season of The Flash has used Cicada and then Cicada II as the big bads, but the mere presence of Eobard Thawne has meant fans wondering what he has up his sleeve. Despite the fact that he's imprisoned and set for execution in the future after seemingly forming a genuine connection with Nora, the man behind the Reverse-Flash mask is Barry's arch-nemesis for a reason. The penultimate episode of Season 5 finally revealed Reverse-Flash's long game, and it used a minor plot detail to pull it off.
Fans have known that Reverse-Flash is behind bars in the future, with his powers dampened enough that he couldn't escape or pose a threat to the people of that time. The focus in his scenes was always on how he was manipulating Nora with his words, but the key to his plan was in front of our eyes all along, and it was the answer to a question few probably thought to ask: how are the future authorities dampening Reverse-Flash's considerable powers?
Cicada's dagger. The weapon that Cicada and Cicada II have used to sap the powers of the metahuman heroes in 2019 throughout Season 5 has been quite literally on Reverse-Flash this entire time. The dagger was strapped to his chest, preventing him from escaping his cell and his execution. Why does this matter? Well, Ralph Dibny of all people figured it out.
Ralph was asking all the right questions, and nobody on Team Flash was really listening to him. During one of the encounters with Cicada II after they separated her from the dagger, she laughingly revealed that she doesn't need it to take down metas. Why? She never had the dagger in the future before she came back to work on her uncle's meta-killing mission in 2019.
As the good guys were all busy trying not to be killed by Cicada II, nobody really cared about this offhand comment... except for Ralph. Team Flash was on the verge of a great victory, having nabbed the dagger and constructed tech that could finally destroy it. Too late, Ralph connected the dots: everything was happening as part of Reverse-Flash's insane and brilliant plan to escape from 2049 prison.
Everything was to set up Team Flash destroying the dagger in 2019. After they did that, his powers could not be dampened by it in 2049, and he could escape. He even has that Negative Speed Force to help him avoid detection and capture. Reverse-Flash is on the verge of breaking out of prison, and it's all because Team Flash is on the brink of their greatest 2019 victory of the season. The finale is bound to be a doozy.
Oh, Reverse-Flash. Never leave, you evil manipulative genius. Well, this explains the episode description for the Season 5 finale next week! It's safe to assume at this point that Reverse-Flash breaks out of prison, and likely zooms back to 2019 to face off against Barry Allen and Co., although the description technically only says that Barry "faces off with his oldest, and most formidable nemesis, Reverse Flash."