In his novel Heart of Darkness, Joseph Conrad centralizes over the theme of the corruption of power into darkness by means of the foreshadowing, parallelism, and irony.
"Look at the influence that man must have. Is it not frightful?"
"get him hanged! Why not? Anything-anything can be done in this country. That's what I say; nobody here, you understand, here, can endanger your position"
"It was the stillness of an implacable force brooding over an inscrutable intention. It looked at you with a vengeful aspect."
This utilizes foreshadowing to precede Kurtz's power as a means of fright and evil.
"The mind of a man is capable of anything-because everything is in it, all the past as well as the future."
This suggests that since there are no rules in "this country" and the imperialists are in charge, that they can do whatever they want with absolutely no consequences, even when their actions are evil.
"They were very big powerful men, with not much capacity to weigh the consequences, with courage, with strength, even yet, though their skins were no longer glossy and their muscles no longer hard"
This uses to parallelism to allude to the way the imperialists brood over the natives and act cruel toward them.
This suggests the evil within man, which can be even more realized when in positions of power over others.
This utilizes irony by setting the power of the men against their undernourishment. It suggests that these men were once powerful here, but since the arrival of the imperialists, even they have been overtaken.