Warning! Contains SPOILERS for Arrow's season 8 premiere.
The season 8 premiere of Arrow may have just killed off two characters from The Flash, with the episode suggesting both Harrison "Harry" Wells (Tom Cavanagh) and Jesse Quick (Violett Beane) are among the first casualties of the Crisis. In the season premiere, Oliver Queen (Stephen Amell) is unknowingly followed to Earth-2 by Diggle (David Ramsay), who insists on helping him with his mission for the Monitor (LaMonica Garrett).
While there, the two reunite with Earth-2's Laurel Lance (Katie Cassidy), who has fully embraced the "Black Canary" mantle. After dealing with Earth-2's version of "The Undertaking", defeating Tommy (Colin Donnell), and retrieving the Dwarf Star particles, Oliver's mission is complete. But before they're able to leave, the Crisis comes to Earth-2 and a wave of anti-matter kills everyone in sight. Oliver and Diggle escape Earth-2, with Laurel in tow.
Earth-2 has been destroyed by the Crisis, taking everyone who lived there with it, including the inhabitants of Gorilla City and Atlantis. What's interesting about this is that Arrow didn't just kill all of the Queen family counterparts introduced in the episode; it also killed characters from The Flash. The destruction of Earth-2 should mean the deaths of both Jesse and Harry. Jesse was a key recurring character in seasons 2 and 3, as well as a member of Team Flash and Wally's love interest. Her father, Harry, was the show's main version of Harrison Wells throughout seasons 2 and 4.
In season 4, Jesse left for Earth-2 and started her own team of superheroes. At the end of the season, Harry parted ways with Team Flash and returned to Earth-2 so that the two could reconcile. Neither has been seen since, so it would seem that the two are still on Earth-2. If that's the case, then Harry and Jesse are both dead, with their deaths rather unceremonious. Of course, The Flash can always bring them back later and offer an explanation for why they survived. It is possible that Harry and Jesse fled Earth-2 prior to its destruction.
The notion that Arrow may have just killed off two major characters from The Flash speaks to the level of connectivity between the shows and how much "Crisis on Infinite Earths" binds them together. Arrow and The Flash are already dealing with the same dilemma, which is the impending death of the main character. As the Crisis continues to unfold, more events may ripple across Arrow and The Flash, affecting both shows in different ways.