Folklore of the 'Nix'

In my novel, the Saxon tribe of the Wolf Sons (who give their name to the book’s title - WULFSUNA) face a terrifying entity. What, you wonder, could put the fear into the hearts of several scores of burly Saxon warriors? Mortal enemies with swords and spears can be vanquished. Those foes do not frighten them. Even death, though often gruesome, is a fate greeted bravely, for it means acceptance to Woden’s great hall and a feast before the last great battle of the world. It is something far more sinister and foreboding that grips these men’s hearts with ice.