From the introduction:
When the Shining Crusade defeated the dread lich Tar-Baphon in 3827 ar, the lich’s lieutenants scattered, hoping to evade the crusade’s triumphant armies. Eshimka, one of the nightwalkers serving Tar-Baphon, fled north to hide and build up its power. Ivvora, a disciple of the divine martyr Vildeis, tracked the evil creature into the Tusk Mountains. There she contacted a nearby tribe of Kellids known as the Twinhorn following, and together, they formed a plan to destroy the nightwalker.