The Breath of God by Harry Turtledove

The second Opening of the World fantasy novel (after 2007's The Gap) can easily be enjoyed on its own terms. The rise of the Rulers, a powerful group who ride mammoths to war and wield potent magic powers, poses a major threat to the Raumsdalian Empire. Count Hamnet Thyssen, a master warrior, rallies a ragtag mix of magicians, shamans and soldiers against the Rulers, who deal them a crushing defeat.