Poul Anderson 
Broken Sword 

The sword Tyrfing has been broken to prevent it striking at the roots of Yggdrasil, the great tree that binds earth, heaven and hell together . . . but now the mighty sword is needed again to save the elves, who are heavily involved in their war against the trolls, and only Skafloc, a human child kidnapped and raised by the elves, can hope to persuade the mighty ice-giant, Bolverk, to make the sword Thor broke whole again. But things are never easy, and along the way Skafloc must also confront his shadow self, Valgard the changeling, who took his place in the world of men.A superb dark fantasy of the highest, and most Norse, order The Broken Sword is a fantasy masterpiece.