The Flash season 5 episode 6, "The Icicle Cometh," has finally revealed the true origin of Caitlin Snow's villainous alter ego, Killer Frost. After finding Caitlin's father, Dr. Thomas Snow (Kyle Secor), Team Flash learned the secrets of Caitlin Snow's past and solved one of The Flash's biggest lingering mysteries.
Killer Frost first appeared in The Flash season 2 as Caitlin's evil Earth-2 counterpart and an ally of Zoom. In The Flash season 3, it was revealed that events of Flashpoint changed Earth-1's Caitlin into a metahuman as well. Caitlin's transformations into Killer Frost worried her to a point where she tried to permanently suppress her Killer Frost personality. After learning how to get along with Killer Frost, Caitlin was devastated when an encounter with DeVoe seemed to erase Killer Frost completely. At the end of The Flash season 4, a flashback revealed that Caitlin Snow wasn't a metahuman, but rather Killer Frost had been a part of her all along.
Caitlin began to investigate her past and decided to go looking for her father, who she had once believed to be dead. After several episodes of chasing clues, Caitlin finally got some answers in The Flash season 5 episode 6, "The Icicle Cometh." Team Flash finds Dr. Thomas Snow in an isolated lab in the north and brings him back to S.T.A.R. Labs. According to Thomas, he was using experimental cryogenic treatment to cure himself of ALS and had no choice but to place himself in quarantine - and he did something similar to his daughter.
Team Flash eventually finds out that Thomas gave himself ice powers like Caitlin's, and that he experimented on Caitlin as a child to prevent her from growing up with ALS. That meant that it was Thomas' experiments that created Killer Frost, not dark matter from the Particle Accelerator.
As it turns out, Caitlin isn't the only one with a split personality. Thomas' alter ego, Icicle, took control and kept the real Thomas from emerging. He wanted to use Caitlin's serum to repress Thomas for good, but his plan failed when Caitlin found a way to bring back Killer Frost... temporarily. Thomas escaped, but one good thing did come from their encounter; Caitlin knows that she can get Killer Frost back, and thanks to Cisco and Harry's mental activity dampener, she now has a way of communicating with her.
This episode of The Flash may have just introduced a new powerful enemy for Team Flash to deal with over the course of season 5. Thomas' alter ego, Icicle, is a villain from DC Comics who was previously featured in Smallville. It's possible that the following episodes of season 5 will see Team Flash battle Icicle in an effort to save Caitlin's father.