Well, the answers you'll most probably get are colonialism and the triangular commerce (aka slave trade).

For the first, in 1885, the Berlin Conference was held and its major purpose was to divide the african continent between the colonial powers of Europe (it was basically divided between Germany, Belgium, Spain, Portugal, France, Great Britain and Italy, while Ethiopia and Liberia remained autonomous).

The colonial era, in Africa, was basically one where European powers took possession of almost all african countries (though Congo didn't belong to Belgium, but rather to the King of Belgium himself, personnally), and were able to use most of their natural resources. It's of course a very complicated subject for which many books are written, but basically, one could argue that if today many countries of Africa are under-developped (some of them are getting better though), it's because of the still lasting effects of colonialism that was in place until well into the 20th century.

For the latter (slavery and triangular commerce), I'm certain someone who knows more in that particular department could help you more effectively than I ever could. It is probably the most impactful event, historically, in terms of how it has affected Africa and its development. But basically, there is more than one single historical event.