The Sea of Trolls

The Sea of Trolls, by Nancy Farmer.

A bard (wizard) shows up at a small farm town, in Angelo Saxon, England. After a year there, he decides to take Jack, a farmer's son, as his apprentice. Jack is learning fast, but not fast enough. Vikings, lead by a beserker, named Olaf-one-brow, takes Jack and his five year old sister, Lucy, as slaves. Lucy is given to the Vikings half troll queen, Frith, (half trolls are shape shifters). Jack accidentally does a spell to make Frith lose her hair, but Jack does not know how to put it back on.

Frith tortures Jack by saying that he is going to kill his sister Lucy, unless he finds a way to put her hair back. But, there is only one way to do that. Jack must find Mimir's Well. So Jack must go on a quest to find Mimir's Well, which can give you any knowledge you desire, but only if you make a great sacrifice...which you have to find in Jotunheim, the land of trolls.

Jack ventures into Jotunheim with the assistance of Olaf-One-Brow and a shield maiden, named Thorgil. As they try to find Mimir's Well, they run into a giant troll bear, which is five times the size of a normal bear. Olaf kills the bear, but not without sustaining mortal wounds. After Olaf kills the troll bear they are attacked by a dragon and taken back to her nest, where the baby dragons are to kill them. Thorgil stabs the baby dragons with her knife, and kills them, while the mother is hunting for food. While she is killing them, some of the dragon's blood goes into her mouth. Which means that she can talk to birds. They make their way to the Troll Queen's palace, where they are treated like royalty. The Norns, people who control destiny, send Jack and Thorgil to Mimir's Well, where they both drink. They come back to the Viking's land one day before the day that Frith was going to kill Lucy. Jack tells her to shave the godesses cats, then to take one third of the hair and to lay down in the moonlight with it around her, but she takes all of the hair and she becomes a scary monster. Then she gets chased by the shaved cats into a bog, where she drowns.