In Joseph Conrad's Heart of Darkness, the title originally refers to Marlow's trip into the portion of Africa once referred to as the Congo, which was "owned" by Belgium. The most profitable export was ivory, and often those involved in collecting it were disreputable men who cares only for the money, and treated the natives ruthlessly.
However, as the story develops, Marlow—who serves as the narrator—is hired by the Company to captain a boat into this "heart of darkness," asked to travel to the Inner Station (the third of three stations) to bring out Kurtz, an extremely successful representative of the Company, who has been cut off from civilization for more than a year.
Hadn't I been told in all the tones of jealousy and admiration that he had collected, bartered, swindled, or stolen more ivory than all the other agents together?
When Marlow finally arrives at the Inner Station, he finds that Kurtz is living in a building surrounded by spikes with human heads on them, and is treated much like a god by the natives. And while Kurtz does not fight leaving the island, the natives are not happy about it.
It would seem that Kurtz's experiences have irreparably changed the man, though Marlow sees reasons for which he might once have admired the other man:
...in his being a gifted creature, and that of all his gifts the one that stood out preeminently, that carried with it a sense of real presence, was his ability to talk, his words—the gift of expression, the bewildering, the illuminating, the most exalted and the most contemptible, the pulsating stream of light, or the deceitful flow from the heart of an impenetrable darkness.
In essence, Kurtz has lost his way. He has seen the darkness in the souls of others; he has seen (and given in to) the heart of darkness lurking within his own soul—perhaps in all men—but instead of resisting it, he embraces it. It destroys the man; it destroys his mind. And in leaving the jungle, Kurtz ultimately dies.
Kurtz's willingness to embrace his own "heart of darkness" leads to his alientation and isolation from his own society, and ultimately draws him from sanity to overwhelming madness. In the struggle between the "light" and the "darkness," it has been a battle that Kurtz could not win. In leaving the jungle, for a coherent moment, Kurtz cries out his final words, which are reflective of what exists with the darkest part of a man's soul; he says:
‘‘The horror! The horror!’’