What two elements left a stamp on the continent of Africa forever?

Imperialism and slavery.

Explanation:

In the mid-1800s, Europe indulged in Imperialism, which resulted in military invasions, conquest, and colonization in Africa. European powers started colonizing for three factors, economic, political, and social. The Berlin Conference in 1885 helped the European powers in the colonization of Africa. Europeans in establishing colonies into the wilderness to get access to natural resources like palm oil, cocoa, and minerals like diamonds and tin to prosper their nations.  

Introduction of slavery in  Europe and the New World disrupted the lives of African people who were uprooted from their country and shipped to a new place as labours. Slavery, for the first time, became part of the trade and led at the beginning of the slave trade.