The ending of the book means some work. You just don’t mark the end. There are some things you need to write. This is a general bit here. We will look at the steps you need to cover to make the ending satisfactory.
First comes the building of steps. These steps are found in the character's motivations, goals and secrets. There are also the stumbling blocks. What the character wants and why he or she wants it? This may or may not be the character's initial goals but ones that have changed and developed. The character's secret is important. He may be hiding something from others and even from himself. For that secret to be revealed is often the trigger for the black moment. Stumbling blocks belong to the other characters and they have their own reasons for the character not to reach his goal. These can also trigger the black moment.
The moment of realization is the next point in the black moment. The character must really believe all is lost. Don't resolve this moment in a sentence or two but show the character really suffering from the realization.
Then comes the resolution. Here the character must give up or find a way to fight. Play the scene out fully as this adds to the drama of the scene. Even if he or she gives up this must be made to look like the right decision for the character. If he or she decides to fight, this will lead to other scenes.
The black moment for your character or characters doesn't just pop up. The moment has to be developed so the reader believes this is for real. Remember during the black moment, the character must believe all is lost. The reader must believe this as well. So how does this moment happen?
Just what is the black moment? this is the time when a character realizes all is lost. The hero knows he will never win the girl. The heroine sees her dreams for happiness evaporate. The villain realizes he will be found out.
The key word is believes and this can be hard to make clear. If you're writing a romance, you know there should be a happily ever after or a happy for now moment. In a mystery, you know the villain will be unmasked. In any genre the time needs to come for one of the characters to realize all is lost. In a science fiction story the hero or heroine might believe the plague will never end. In fantasy, the wizard believes he or she ahs lost their powers. A bit of note for the horror genre. The black moment may become the end of the focus character, What could be blacker than that.
Is it possible to have more than one black moment in a story? Skillfully done, this can be achieved. The character may believe all is lost. Then he or she decides to go on from there. Then, during the new path he or she has taken there comes another time when all can be lost.
Here’s a sketch of what my Black Moment will entail. The hero has tried to rescue one of the girls the villain holds prisoner. Instead he is captured and the villain taints his lines. The hero believes he has lost everything and he flees. When the alien finds him, he is unable to fight the man. The heroine arrives. The alien is pleased but the heroine offers the hero a chance to cleanse his lines. He must decide to do something he has fought since he first met the heroine. This leads to the final scene and also gives a little hint about what the third book in the trilogy will be.
So your assignment is to spend time trying to figure which one of your characters will have the really black moment. In my story though the heroine has to make a hard choice it is not a true black moment for her.