The Nigerian author Chinua Achebe criticized Joseph Conrad's novel, Heart of Darkness, as being racist. He did so in a lecture, later published, at the University of Massachusetts entitled An Image of Africa: Racism in Conrad's "Heart of Darkness. In his lecture, he criticizes the way Conrad portrays blacks (Africans) in his novel and takes exception to his over-use of the derogatory word "nigger" in his writing. Achebe also acuses Conrad of stereotyping blacks - using very pejoritive words to describe them, thoroughout the novel. He also says that Conrad is obsessed with "blackness" because of all the dark imagery in the novel.
Other critics have criticized Achebe for criticising Conrad. They accuse Achebe of being overly-sensitive and carrying political correctness to the extreme. They claim that Achebe cannot separate his own racial views from Conrad's art.
You will need to research this controversy and decide on the thesis for your own essay. You could research Achebe's criticism and write your essay either agreeing or disagreeing with it. Or, you could do a compare and contrast essay, presenting both sides. There is information available online regarding this controversy and you should also read the analysis of Heart of Darkness here on enotes. Also, there is a good deal of information about Achebe here on enotes at the link below.