Revival is the episode where they have to revive Chelsea or else it's game over and the darkness will consume the entire planet.
the episode is set some time after the end of the series, where a Magical Terra Destroyer threatens to destroy the world. but he says if they revive Chelsea, he will leave their planet alone. to save their planet from its doom, Akame and the rest of the Night raid try to figure out a way to revive Chelsea or it's game over.
After a numerous amount of failed attempts, in the last attempt,succeed in reviving Chelsea. because they revived Chelsea, it meant that the Magical Terra Destroyer will leave the planet alone. and as a result, saving everyone from certain death.