Although The Long Winter was hard for everyone, smaller races such as the humans and Dwarves were better at adapting than many other ancient races. Two races of humans came out of the Ice Age. Some resembled modern man, and others were of a more primitive breed. Brute Men they were called, and what they lacked in wisdom they made up for with physical strength. But it was their weak minds that would be their undoing.
During this age, a human shaman named Keres became fascinated with Death. Using his spirit magic, this Shaman was able to lure a God named Temrin into this world. Wielding a weapon that had been trapped in the Ice since the Cthonic Age, Keres murdered Temrin, drawing his power into himself. Upon the site of Temrin's murder would later be built a construction called the Temple of Id.
Keres the Shaman, used the power he had drained from Temrin to awaken the ancient Sleeping Ones trapped deep beneath the ground. Although they were still trapped, these Burrowers were slowly beginning to dig their way out. But even without reaching the surface, their minds could travel much faster.
In the north, both Humans and Brute Men had only mastered tools of stone, but the superior intellect of Humans allowed them to withstand the mental corruption of the Burrowers. Many of the Brute Men, however, became the unwilling slaves of those evils below. These corrupted Brute Men began to wage wars on Humans and Brute Men alike.
Raging on for two centuries, the Burrower Wars brought unthinkable destruction to the world. Even Dragons, usually oblivious to the affairs of the lesser races, were drawn into action. As a powerful Burrower known as Iscranin was about to emerge near the North Pole, it was attacked by the Great Dragon Yealaletherveri. Yealaletherveri died, but only after preventing the Burrower's escape.
The Gods now realized how serious the situation was. Learning of the murder of Temrin, it was agreed that intervention into the Mortal World was permitted. The Burrowers were returned to deep sleep. This marked the end of Brute Men in the North. The Burrower Wars had also weakened the position of Dragons, who were soon driven into the mountains by Giants. The Age of Men was still centuries away.
What happened to Keres the Shaman is not known. Some say he was punished by the Gods. Others say he had become a God himself.
Edit: See also Garls of Blackmoor here
Behind the curtains: Did I make up all of this?
What I like to do is combine information from different sources. The Burrower Wars were introduced in the Hollow World Boxed Set and expanded upon by OldDawg. I decided to combine this with the murder of Temrin as described in Dave Arneson's Blackmoor D20CS as well as elements of the history of Blackmoor from Dungeons of Castle Blackmoor (such as the Cthonian Age). Iscranin and Yealaletherveri appear in the novel Son of Dawn and are placed into context in the History of Dragonkind by Simone Neri.
More discussion of this article here.