Arrow season 8 kicked off Crisis On Infinite Earths with a literal bang - by destroying Earth-2. The CW's namesake flagship series is now in its final season building towards this year's massive Arrowverse crossover. After the events of Arrow season 7, Oliver Queen (Stephen Amell) left his world, Earth-1, behind to join the Monitor (LaMonica Garrett) in a quest to prevent the destruction of the Multiverse. But the Crisis has already begun and claimed a major casualty in Earth-2.
Crisis On Infinite Earths is an event with the fate of the entire Multiverse at stake but on a personal note for the Green Arrow, the cosmic Armageddon results in his death according to the Monitor. Believing his demise is a fait accompli, Oliver is now performing missions for the mysterious cosmic being in the hopes of being able to stop the Crisis from destroying billions of worlds. In the Arrow season 8 premiere, "Starling City", Green Arrow traveled to the Earth-2 version of his home city, where he had been missing on the island of Lian Yu for 12 years instead of 5. Oliver's quest was to acquire dwarf star technology from Queen-Merlyn, the consolidated company formed by his mother Moira (Susannah Thompson) marrying Malcolm Merlyn (John Barrowman). Speaking of dead characters who are alive on Earth-2, Oliver also discovered Adrian Chase (Josh Segarra) is the new Hood in Earth-2 (mysteriously replacing Robert Queen) and his best friend Tommy Merlyn (Colin Donnell) is the Dark Archer.
While Oliver accomplished his mission, "Starling City" ended with a tragedy Green Arrow could do nothing about: an antimatter wave engulfed Earth-2 and destroyed the entire planet. Oliver watched as Moira and Tommy were disintegrated by antimatter and he barely escaped thanks to the device invented by Cisco Ramon (Carlos Valdes) that allowed John Diggle (David Ramsey) to jump to Earth-2 to join Oliver. As far as fans are aware, the lone survivor of Earth-2's destruction is Laurel Lance (Katie Cassidy), who is the reformed villain Black Siren and had taken up the mantle as that world's Black Canary.
Earth-2's doom was actually foreshadowed on this week's episode of The Flash, "A Flash of the Lightning". Barry Allen (Grant Gustin) sought the help of Earth-3's Jay Garrick (John Wesley Shipp) to find a way to see the Crisis for himself; the Flash was able to mentally run into the Crisis but he witnessed the destruction of billions of realities by the antimatter wave, including the deaths of his friends and his wife Iris West-Allen (Candice Patton). When Garrick showed Barry a map of the Multiverse he made, it showed a red arrow pointing to Earth-2, which would be the site of the next major antimatter incursion - which is exactly what happened on Arrow.
Team Flash first visited Earth-2 in The Flash season 2, episode 13, "Welcome to Earth-2", which properly opened the door to the Multiverse, which includes Earth-38, the home of Supergirl (Melissa Benoist) and even the Nazi-controlled Earth-X. Earth-2 is the home of Harrison Wells (Tom Cavanaugh) and his daughter Jesse (Violett Beane), who became the speedster named Jesse Quick. Earth-2 has several notable differences from Earth-1, including a notable golden hue to its environment, and Gorilla City could be found hidden in Earth-2's African jungles. Atlantis also exists in Earth-2, which opens the door to the possibility of Aquaman defending that world's oceans. Thanks to the sight of Batman's mask on Lian Yu and Adrian Chase quoting Bruce Wayne on Arrow, we also now know that Earth-2 has its own Batman.
It's not yet known if anyone else besides Laurel Lance survived Earth-2 destruction; Laurel got to live so she can join the Dinah Drake Black Canary (Juliana Harkavy) and Mia Smoak (Katherine McNamara) in a potential Arrow spinoff. However, losing Earth-2, which was the site of many memorable adventures on The Flash, is a truly tragic event and it's an effective and memorable way to let Arrow hammer home just what's at stake in Crisis On Infinite Earths.