In OTL horses couldn't live in Sub-Saharan Africa except in some narrow bands of territory in the Horn, Sahel, and South Africa. This is because of Sleeping Sickness carried by Tsetse flies which lays waste to horses. Now to an extent this helped the Bantu peoples by holding off Eurasian...
Sub-Saharan Africa had some pretty significant and sizeable empires, but nothing on the level of Persia, Rome, China, the Arabs or the Incas. Your challenge is to have sub-Saharan Africa produce an empire on a truly grand scale.
Your challenge is to have a sub-Saharan African empire expand into Europe and hold some European territory. It doesn't have to be for very long, it just has to happen.