The adaptation of Elseworlds, where Oliver Queen (Stephen Amell) and Barry Allen (Grant Gustin) switched places. However, this crossover also revealed what would come next.
the Anti-Monitor from eliminating everything outside of the antimatter universe. While they were eventually successful in stopping him, it did come at a price as Flash and Supergirl from the prime Earth died during the event. They weren't the only casualties though, as alternate heroes, villains, and, in some instances, entire realities were lost.
While speaking to five-hour crossover story will start with several parallel universes being destroyed.
Basically, we’re gonna begin — this is an exclusive — we’re gonna begin the way Crisis on Infinite Earths the comic begins, which is the destruction of various parallel universes. And the goal is for us to adapt key moments from the comic, those seminal moments.
The destruction of multiple universes will be one way to instantly kick the Crisis on Infinite Earths event off with a fitting mood and help establish what is at stake for the heroes. But, as more universes are destroyed, that will explain how heroes escaping from them all come together. Based on Guggenheim's answer, they are trying to do their best at recreating and reimagining several major events from Crisis on Infinite Earths and won't just settle for an opening like the comics.
He later teased in the interview that his pitch to The CW for the event included showing them various covers from the comic event and explaining how they would do the Arrowverse version of them. This could add credence to the theories that Brandon Routh is going to be Superman again in the crossover. With many of the Arrowverse shows set to have s at SDCC today, it may not be long before we learn even more details about how they'll build up to Crisis on Infinite Earths.
Source: Comicbook