Finally, on The Flash, Barry Allen and his team have been dealing with a metahuman serial killer they call Cicada. Cicada was created in the Season 4 finale, during the Thinker's Enlightenment. When Team Flash crashed Thinker's plan by destroying his satellites, they caused chaos throughout Central City. The debris landed at a local fair, injuring Cicada's niece and wounding Cicada himself; because the satellite was saturated with dark matter, the resulting injury gave Cicada a wound that can't heal, a dagger that can dampen other metahumans' powers and a few metahuman abilities of his own, including flight. Now, Cicada has made it his mission to kill every metahuman, as he believes they are the source of Central City's problems.
Cicada isn't the only disruption in Team Flash's lives. Nora West-Allen, Barry and Iris' full-grown daughter from the year 2049, has traveled back in time in order to get to know her father better. According to Nora, Barry disappeared in a "Crisis" in 2024 and has been gone ever since, meaning Iris raised her as a single mother. Now, Nora in Barry's present day learning how to use her speed powers under his tutelage, even as he struggles to come to terms with his future.
Even though Nora is a series regular this season, don't expect to see her in the crossover. "I am not in the crossover episodes," Nora actor Jessica Parker Kennedy told CBR. "I know! But I got to be such a huge part of the 100th episode, I feel like I can't complain at all." This is probably due to the fact that Barry and Oliver have switched lives, meaning that Oliver would have been the father and not Barry.
As to the rest of Team Flash, Caitlin Snow has seen the biggest transformation. Though the Thinker appeared to erase Killer Frost by suppressing Caitlin's meta gene, that wasn't the case at all. As it turns out, Killer Frost was born when Caitlin's father used experimental cryogenic gene therapy on her in order to prevent her from developing ALS; the Thinker simply used telepathy to bury Killer Frost's personality deep down in Caitlin's brain. After Caitlin's father resurfaced, Caitlin not only learned the truth about Killer Frost's origin, but also that her alter-ego still exists within her. As such, Team Flash has developed a way for Caitlin to call upon Killer Frost as needed. Better yet, Killer Frost isn't affected by Cicada's dagger, making her a key weapon in their fight against him.
Season 5 also has a new Harrison Wells in the mix. This year, it's Sherloque Wells, who calls himself the mutliverse's greatest detective. So far this season, he has worked with Ralph Dibney and Cisco "Vibe" Ramon to determine Cicada's identity. He also appears to be extremely suspicious of Nora, who -- as revealed in the series' 100th episode -- has been working with Reverse-Flash in the year 2049.
This year's Arrowverse crossover will involve the casts of Supergirl, Arrow and The Flash. The event will unfold over three nights, starting with a special episode of The Flash on Sunday, Dec. 9, at 8 p.m. ET/PT. It continues Monday, Dec. 10, with Arrow, and concludes on Tuesday, Dec. 11, with Supergirl.