The Wandering Fire — Guy Gavriel Kay

The ice of eternal winter has reached out to enshroud Fionavar, the first of all worlds. For the Unraveller has broken free after millennia enchained--and now his terrible vengeance has begun to take its toll on mortals and immortals, mages and warriors, dwarves and the lios alfar, the Children of Light.

And only five men and women of our own world, brought by a mage's power across the Tapestry of worlds to Fionavar, the very heart of the Weaver's pattern, can hope to wake the allies they so desperately need. Yet none can foretell whether even these beings out of legend--from the mysterious Warrior of prophecy and dreams to the riders of the Wild Hunt-have the power to shatter the Unraveller's icy grip of death upon the land....