SPOILER WARNING: This article contains major spoilers for "What's Past is Prologue," the 100th episode of The Flash.
Killer Frost might be the key to defeating Cicada. In "What's Past is Prologue," the 100th episode of The Flash, the team attempted to stop Cicada once and for all. However, their plan fell through and he was able to use his power-dampening dagger to stop them all -- except for Killer Frost, that is.
At the beginning of the episode, Team Flash deduced that the only way to stop Cicada was to stop him from using his dagger. In order to do that, they needed a strong alloy with magnetic properties, irradiated with a similar amount of dark matter. Naturally, they didn't want to risk another particle accelerator explosion, so they decided to head back to the event that gave Barry his powers. Along they way, they picked up a few items they needed to make it work, like a piece of Savitar's armor and Zoom's Speed Force transmitter.
When they assembled all the pieces, Team Flash successfully built an object that negated Cicada's dagger and went to confront him. After they employed the magnet, Cisco was able to vibe the dagger into deep space. As soon as Cicada no longer had his dagger, Team Flash was able to land a few punches. Unfortunately for them, however, he was able to call the dagger back to himself despite the distance. With his dagger back in play, he was able to toss Flash, Vibe, Ralph and XS aside like rag dolls.
When Cicada attempted to kill Barry, Caitlin panicked, which brought Killer Frost out. Cicada turned the dagger towards her, but it had no effect. Uninhibited, she used her Killer Frost powers to blast him backwards, stunning him. She turned to her friends to make sure they were okay, but -- when she went to apprehend Cicada -- he managed to escape by flying off into the night.
Killer Frost's origin, as revealed in "The Icicle Cometh," may explain her resistance to Cicada's dagger. As it turns out, Caitlin manifested Killer Frost when her father attempted experimental cryogenic gene therapy on her to prevent her from developing ALS. As such, her powers don't stem from dark matter, as is the case with the rest of Team Flash. It seems, then, that Cicada's dagger -- which was created by dark matter -- can affect only those whose powers also derive from dark matter.
Considering that, Killer Frost may be the only person who can use their powers around Cicada and his dagger. This makes her an incredibly dangerous foe for Cicada, who has thus far relied on his dagger to protect himself. In fact, at this point, she may be the only character on the series who can take him down.
Airing Tuesdays at 8 p.m. ET/PT on The CW, The Flash stars Grant Gustin, Candice Patton, Carlos Valdes, Danielle Panabaker, Tom Cavanaugh, Jesse L. Martin, Danielle Nicolet, Hartley Sawyer, and Jessica Parker Kennedy.