And if you don't know.
The Darkest Hour can be defined as when the villains of the story "win." Moreover, the reason as to why "win" is in parenthesis is because they don't win the whole story, but it's a very major victory.
Some examples are they manage to kill a major partner of the main hero's allies, there parents, or even turn everyday citizens against them.
So, can you name some. Oh and be sure to put down your spoiler tags for people who still haven't read that particular manga/anime.
I shall go first.
Yu Yu Hakusho