In the original timeline, the Crisis took place on the night of April 25, 2024, where the Flash fought his nemesis, the Reverse-Flash, with the help of his allies in an extreme street battle in Central City. The fight ended with the speedsters disappearing in an explosion of light.

Does The Flash actually disappear?

At the end of The Flash's Season 6 premiere, The Monitor told Barry Allen that he “must die.” Instead of dying in 2024, Barry would instead vanish December 10, 2019. And so, the Scarlet Speedster spent the remaining episodes ahead of “Crisis on Infinite Earths” preparing Iris and his teammates for his demise.

Why must The Flash die?

In the Flash season 6 premiere, the Monitor assures Barry that in order for the multiverse to be saved, he must die. ... In order to prevent the antimatter from destroying the entire universe, Barry ran faster than ever before to counteract the antimatter and was thus swallowed into the Speed Force.