Content appropriate for teenagers to young adults

“The horror! The horror!”

When Marlow, an experienced sailor, goes to Africa as the captain of a steamboat on the Congo River, the journey has a profound effect on him. As Marlow sails further down the river and into the heart of the African continent to bring back ivory and a brilliant but mysterious agent called Mr Kurtz, he feels he is travelling further and further into the heart of darkness.
What is this darkness? What powers of darkness take Mr Kurtz?