In the fifth century, Attila the Hun terrorised both the Eastern and Western Roman Empires; his were the first of the Devil's horsemen. After his death his empire quickly disintegrated - but then came the Magyars, fighting the early Holy Roman Empire, and the heavy cavalry used to counter them is where the concept of the knight in armour comes from. Thirdly, there are the Turks, the only group that didn't expand into Europe, but instead conquered the Middle East. Like the other groups, they were shamanistic pagans and yet within a generation they had converted to Islam.
Fourthly, and most famously, we have the Mongol Empire, the largest land-based empire in history, spreading from Korea to Poland. It was hugely important for a century and its impact lasted for half a millennium. Finally, there is Tamerlane. Alexander the Great gets a lot of credit for fighting for eight years without a loss. Tamerlane fought for 25 without loss and he was paralysed down one side of his body. He would even vanquish two of the earlier groups, in wars against the Mongols and the Turkish Ottoman sultans.
What links them all is the horse as a weapon of war. In his own attractive style - 'as accessible as it is informative' (Deus Vult) - Jem Duducu rides across a thousand years of conquest on horseback.