Hermod and Hadvor
Hermod and Hadvor is an Icelandic fairy tale.  Andrew Lang included it in The Yellow Fairy Book.

Synopsis
A king and queen had a daughter, Princess Hadvor, and a foster son, Prince Hermod. One day, the queen died. The king set to sea and found a beautiful woman with another, younger woman, both finely dressed, and a third woman, their maid. This woman told him that she was the Queen of Hetland and had been driven from her land. They married. 

Hadvor and Hermod took little note of her stepmother, but Hadvor became friendly with the maid, Olof. The king went to war, and the new queen, who was a wicked witch, told Hermod that he had to marry her daughter, also a witch. He refused, and the queen cursed him to go to a desert isle and be turned to a lion by day and a man by night, never to be freed until Hadvor burned his skin. He cursed her to, as soon as he was free from the enchantement, become a rat and her daughter a mouse, and they would fight until he killed them.

Olof told Hadvor what had happened, and that the queen wanted her to marry a three-headed giant in the underworld, whom the queen would turn into a handsome prince. When this giant arrived, he would come through a hole in the floor; if Hadvor had plenty of burning pitch ready, she could pour it in. The giant came, and Hadvor killed him by pouring the pitch on him. The queen found the body and laid a spell, so that it became a handsome prince, and Hadvor was unable to defend herself from any charges. The king agreed to let the queen punish Hadvor, but Olof warned her.

Hadvor made her way to the sea. In a dream, a woman told her that she was leaving her a rope to climb the cliffs, a thread to follow to find Hermod, and a belt to keep her from going faint with hunger. She found these when she woke. With them, she found a cave. Hermod came there in the evening and shook off his lion skin; Hadvor burned it. A good witch with fifteen sons gave them a boat to get back, and warned them that the a whale would drown them, but they could call on her name for aid. When they got to sea, the whale did attack, but when they called on the witch it was attacked by another whale, plus fifteen smaller whales, and they got home. 

They found there the rat and the mouse, and Hermod killed them, destroying both evil witches. They told her father the whole story. Her king married the two of them, and resigned his kingdom. Olof married a nobleman.

External links
Hermod and Hadvor