A race of monsters dwelt for generations below the ground at Ikos in the Elabonian Empire. On one horrific occasion, an earthquake let a large population of these monsters into the human world, touching off a massive conflict.
The cover of Prince of the North depicts a creature that resembles a hybrid of a bear and a Minotaur and/or a Krampus. This appears to be an incorrect interpretation of Harry Turtledove's text. The description in the novel suggests that the monsters are fairly ordinary hominid beings, possibly Neanderthals.