8. Crisis on Earth-X, Part 2

Complete story — includes the ending

In S.T.A.R. Labs, a captured inter-dimensional villain from Earth-X, Prometheus, is revealed to be a parallel universe doppelgänger of Tommy Merlyn. After he cruelly taunts Oliver over his attachment to Earth-1's Tommy, Prometheus commits suicide out of loyalty to Earth-X's Nazi regime. Prometheus's partners, Dark Arrow (Oliver's doppelgänger), Overgirl ( Kara Danvers ' doppelgänger), and Eobard Thawne steal a sublight generator, the Prism, from research company Dayton Optical Systems; Oliver, Barry Allen, Kara, and the others believe that Dark Arrow seeks to build a neutron bomb with it to aid in his conquest of Earth-1. After a series of battles, most of the heroes—including Team Arrow—are held captive at S.T.A.R. Labs after Nazi forces infiltrate it. Oliver, Barry, and his allies are abducted to Earth-X's concentration camp, but Kara is moved to S.T.A.R. Labs. Oliver is determined to kill Dark Arrow after his capture. The villains reveal that Overgirl is dying and Dark Arrow plans to use the Prism, charged by S.T.A.R. Labs' particle accelerator, to produce artificial red sunlight and remove Kara and Overgirl's invulnerability, allowing a heart transplant for Overgirl but killing Kara in the process. Note: This episode continues a crossover event that begins on Supergirl season 3 episode 8, continues on The Flash season 4 episode 8, and concludes on Legends of Tomorrow season 3 episode 8.