This is the part of the the story when Kurtz dies. His last lines are "The horror, the horror." This is when he realizes the darkness that is in the people , and he also realizes the darkness that was in him.
This is when Marlow returns to Europe, and he tells Kurtz's fiance that he has died. He sees how she is blind to the darkness, and thinks that he is super wonderful.
This is the ending of the story, where Marlow is wrapping up his tale to the other sailors.