Elseworlds was one of the cooler projects DC Comics did. The idea was to let their writers off the leash and imagine new and bizarre versions of their universe. As such, it gave us classic stories like Batman: Red Rain, Kingdom Come and Superman: Red Son, before being left dormant in 2003 (there was a brief resurrection in 2010, but that only saw one title released under the imprint). It seems that the Arrowverse producers haven't forgotten about it, though, as it's now being used for what's described as their biggest crossover event yet.