Crisis on Infinite Earths in the comics concludes with a battle of epic proportions. Heroes unite from different realities and, in the end, the fabric of the multiverse is altered in ways that have long-lasting effects on DC Comics.
The main characters would likely include Oliver Queen, also known as the Arrow. He's central to the Arrowverse and would play a major role in a Crisis on Infinite Earths fanfic. Other important characters could be Barry Allen, the Flash, as his powers and his role in the multiverse are significant. And of course, Kara Zor - El, Supergirl, who has her own unique abilities and would be involved in the multiversal events.
One of the main storylines is the multiverse in peril. Many different Earths from the DC universe are at risk of being destroyed. Another is the gathering of heroes from various Earths to try and stop the destruction. For example, heroes like Superman, Batman, and Wonder Woman from different versions of their respective Earths come together, each with their own unique abilities and challenges they bring to the fight against the cataclysmic event that threatens all of existence.
The main way it ruined DC Comics was that it threw the established order into chaos. It made it hard for new readers to jump in and for long-time fans to keep up. Plus, it seemed to sacrifice character development for the sake of a big event.
In Crisis on Infinite Earths comics, not many Supermen die. Usually, it's a limited number, and the focus is more on the overall story and multiverse chaos.