Beyond the Gap
Harry Turtledove
 When a gap opens in the glacier, Count Hamnet Thyssen and Ulric Skakki are dispatched by Emperor Sighvat II of Raumsdalia to explore the other side. Together with Earl Eyvind Torfin and a wizard, Audun Gilli, they team up with Trasamund, a chieftain of the nomadic, mammoth-herding Bizogot nation. Crossing through the gap, the explorers discover that a powerful tribal confederation, who call themselves "the Rulers," are preparing to burst through the gap and seize the lands to the south for their own.