This week's episode of The Flash predicted the destruction of Earth-2, which came true immediately afterward on the Arrow season 8 premiere. Both series are in the thick of setting up Crisis On Infinite Earths, this year's massive crossover that will also span Supergirl, DC's Legends of Tomorrow, and Batwoman as well as bring in Tom Welling's Superman from Smallville and Brandon Routh as Kingdom Come Superman. All of the superheroes will have to unite to save the Multiverse from a cosmic destroyer called the Anti-Monitor (LaMonica Garrett).
In the Arrow season 8 premiere, "Starling City", Oliver Queen (Stephen Amell) was brought to Earth-2 by the Monitor, who Green Arrow is working alongside in a desperate bid to try to prevent the coming Crisis. Oliver's task was to retrieve dwarf star technology, which was complicated by his meeting his loved ones who are dead on Earth-1, including his mother Moira (Susannah Thompson), his best friend Tommy Merlyn (Colin Donnell), and Malcolm Merlyn (John Barrowman). On Earth-2, Malcolm and Moira are married while Oliver's sister Thea (Willa Holland) is dead. What's more, Tommy is the villainous Dark Archer while Adrian Chase (Josh Segarra) is the Hood of Earth-2, working alongside the reformed Laurel Lance/Black Canary (Katie Cassidy). Though Oliver's mission was successful, the episode ended with a horrible tragedy: Earth-2 was completely destroyed by an antimatter wave, which only Oliver, Laurel, and John Diggle (David Ramsey) were able to escape by jumping back to Earth-1.
However, eagle-eyed fans who watched The Flash an hour before caught the set up for Arrow's Earth-2 annihilation. In "A Flash of Lightning", Barry Allen (Grant Gustin) traveled to Earth-3 to meet with Dr. Jay Garrick (John Wesley Shipp). Barry tried to time travel to December 10, 2019, to see the Crisis for himself but he was repelled and injured. Garrick showed him how to make the trip mentally, not physically, and the Flash of Earth-1 experienced the deaths of billions of realities as well as his own demise. But before Jay sent Barry's mind into the Crisis-filled future, he showed the younger Flash a map of the Multiverse that he made. According to Garrick, after a year of charting antimatter signatures, he pinpointed the next incursion - and a handy red arrow pointed directly at Earth-2!
Unfortunately, Barry never registered the imminent threat to Earth-2 and he didn't take any steps to warn Harrison Wells (Tom Cavanagh) that the antimatter wave was headed for his world. Although to be fair, Jay only showed Barry his Multiverse map for a couple of seconds, and Garrick was more concerned with solving Barry's immediate problem. But this lapse in judgment by the Flashes meant that as far as fans know, no one on Earth-2 saw the antimatter wave coming and it's possible Laurel Lance is her world's only survivor, with Wells, Jesse Quick (Violett Beane), and everyone else on Earth-2 like the denizens of Gorilla City incinerated by antimatter.
Arrow's producers needed permission from The Flash's creative team to destroy Earth-2 seeing how the alternate Earth was introduced by The Flash has been the site of many memorable adventures for Barry Allen and his friends. But this type of synergy is typical of the Arrowverse shows and one of the reasons why it's the most successful superhero universe on television. Arrow and The Flash, especially, have seamlessly worked together since the inception of the Scarlet Speedster's spinoff. The Arrowverse was built on how fluidly characters and events interface and influence each other, which made their annual crossovers possible.
For Crisis On Infinite Earths, the consequences are truly Multiversal - evidenced by how the Monitor leaps from Earth-1 to Supergirl's Earth-38 to weave all of the various series together. The Arrowverse has been building towards the Crisis literally since the series premiere of The Flash but with the mega-event mere weeks away, Arrow immediately ramped up the stakes by literally wiping the popular Earth-2 off the map. And, since The Flash predicted Arrow's shocking Earth-2 armageddon, it means that fans are behooved to pay attention to every Arrowverse show to see how each series builds off the other as they speed towards Crisis On Infinite Earths.