Sister Light, Sister Dark — Jane Yolen

"And you two shall be sisters. You shall be as images in a glass, the one reflecting the other...."

Jenna was a warrior.

Raised on a mountainside, she learned the arts of the warrior, and from the mountain woman the magic of the ancient lore. But the greatest magic of all was her ability to call forth her dark sister from the depths of the mirror of the land of light and shadow.

Skada was the dark one, able to exist only when the moonlight cast a shadow or lamplight flickered in a darkened room.

This is the story of Jenna and Skada. Sister Light and Sister Dark...