The darkest moment in a story is when the character has finally run up against a brick wall. There appears no solution to his problem, no resolution, and no way out.
The darkest moment is essential for generating conflict in any story. It also provides a turning point for the character. Even happy stories need a dark moment, for if everything is running smoothly, there is no incentive for the reader to continue the story to find out what happens.

How’s He Going to Get Out of This One?
There is a B movie called Deep Rising where the main character, when faced with a series of new and life-threatening situations, after each challenge cries out in exasperation, “Now, what?”
A well-crafted story contains a series of progressively worsening setbacks and obstacles for the character to overcome, moments in which the character might very well scream in utter frustration, “Now what?”

These series of challenges are dark moments, but not the darkest moment. A writer must keep throwing hurdles at the character. Never let them get too comfortable. The protagonist’s situation should go from bad to worse to seemingly impossible before the character succeeds. There should be a point in the story where the reader asks, “so how is he going to get out of this one?
Getting Your Character Deeper and Deeper into Trouble
Let’s take, for example, the simple story of a shipwreck. First, the ship begins to sink, and a lone survivor is forced onto a raft. He floats for several days in the blinding heat, no food or water, seeking rescue. Suddenly, he spots shore. A storm is coming which makes it dangerous to swim for shore, but he has no other choice because his raft has just broken apart. It can’t get any worse, can it?

Here Come the Sharks!
It just got worse. The character is about to reach shore, when here come the sharks. Just when he is at his weakest, has been physically and emotionally challenged past the breaking point, he must face the most trying challenge of all.

Throw in a Psychological Fear
To make matters worse, ever since seeing Jaws when he was a child, our hero has had an irrational fear of sharks. How will he be able, in a weakened physical conditon, to confront his worst childhood fear and find a way through this row of hungry mouths and evil teeth to safety?

Once the hero has gone through a series of challenges worthy of Hercules, he begins to grow as a person. Maybe he finds an inner strength he never knew he possessed. Maybe he realizes he is stronger than he ever imagined he could be; maybe he realizes his fear of sharks was irrational.
Only when a character has suffered and made sacrifice, only after he has proven himself worthy by completing an emotional and physical journey from here to there, is the reward well-deserved and the reader satisfied.