This is the deepest, darkest, scariest part of the journey, but it's also the most redemptive. This is where your hero faces their greatest fears. If your hero is scared of snakes, this is where the snakes show up. Of spiders, there'll be spiders. Of commitment, this is where the hero will have to commit. Of death or love or family. Whatever that fear is, whatever it is - physical or emotional fear - you've convinced your reader that scares your hero to death, this is where that shows up.
And this is where your hero is going to have to deal with those fears. This is where your hero absolutely needs to be in physical jeopardy of their life or in metaphorical jeopardy of the same intensity. This may be the most intense section of your story - where the hero faces the fears and learns that he or she can conquer them. This is where your hero realizes - finally - that he or she is capable of carrying out the quest that began the story.
The hero may not believe they can win, but they do believe they can do it, they can get there and fight the dragon. Whatever else happens here, the hero will walk away with a sense of their own ability, their own self worth.
So there are three things that are crucial at this point:
1. The deepest darkest fears have to show up.
2. The hero has to realize that they can cope with those fears.
3. The hero will gain something, and the best something will be both the physical goal (killing the dragon) and an emotional goal (I didn't believe I was brave enough, but I was).
This is the central point of your book, the place where everything begins to come together. This is where you have the darkest moment, the scariest place that leads to the prize.