Nazis from Earth-X invaded tonight during the first night of the Arrowverse crossover "Crisis on Earth-X", and it turns out that they have a far more sinister reason than you might expect.
Spoilers below for part two of "Crisis on Earth-X" below!
When the Earth-X Nazis invaded Earth-1 tonight, the reason for their very unwelcome visit wasn't clear. Were they there to take over Earth-1? Were they displaced from time? While it was pretty quickly sorted out that they weren't time displaced and that taking over Earth-1 was definitely on the short list of things they hoped to accomplish, the real reason for their arrival revealed itself near the end of the second hour of the crossover.
Early in the second hour, the Earth-X villains steal a prism from Dayton Optical Systems. While Team Flash -- specifically Harry -- determined that the object could carry out a stable fusion reaction and thus be converted to a neutron bomb, later in the episode we discover the real reason the bad guys wanted that prism. You see, Earth-X's evil Supergirl is dying. She's been exposed to too much solar radiation, and it's causing her heart to fail her. In order to save her life, they need to take the heart from another version of Kara Danvers, and while there may be 53 versions total, the one presently hanging out on Earth-1 is the one they have in front of them.
With the Nazi's having defeated the heroes, they're all chained up with some sort of device around their necks rendering them powerless, and that's when the true mission is revealed: they want to use the prism to weaken Supergirl so that they can cut out her heart and give it to Overgirl. And while she and her husband Dark Arrow will take the heart no matter what, they are going to give Kara a chance to simply cooperate -- by using the other heroes and her sister as leverage. Next thing the heroes know, they're imprisoned in a concentration camp on Earth-X, and Kara? She's nowhere to be found.
"Crisis on Earth-X" is a four-hour event that will air on November 27th and 28th beginning at 8 p.m. and running for two hours each day; Supergirl (8 p.m. Monday), The Flash (8 p.m. Tuesday) and Legends of Tomorrow (9 p.m. Tuesday) will remain in their normal time slots but Arrow will move from Thursday to a 9 p.m. Monday placement for the week.