Crown of Ice - Mirror of Immortality 3rd edition

Snow Queen Thyra Winther is immortal, but if she can't reassemble a shattered enchanted mirror by her eighteenth birthday she's doomed to spend eternity as a wraith.

Armed with magic granted by a ruthless wizard, Thyra schemes to survive with her mind and body intact. Unencumbered by kindness, she kidnaps local boy Kai Thorsen, whose mathematical skills rival her own. Two logical minds, Thyra calculates, are better than one. With time rapidly melting away she needs all the help she can steal.

A cruel lie ensnares Kai in her plan, but three missing mirror shards and Kai's childhood friend, Gerda, present more formidable obstacles.

Thyra's willing to do anything--venture into uncharted lands, outwit sorcerers, or battle enchanted beasts--to reconstruct the mirror, yet her most dangerous adversary lies within her. Touched by the warmth of a wolf pup's devotion and the fire of a young man's love, the thawing of Thyra's frozen heart could prove her ultimate undoing.