The Heart of Time: The Guardian Heart Crystal Series

Each time the crystal has appeared, regardless of the world or time, its guardians have always been ready to defend it from all who would use it selfishly. One girl stands in the center of these ancient guardians, the object of their affections, and she holds within her the power of the crystal itself. This is the bearer of the crystal and the source of its power. The lines, however, blur and guarding the crystal slowly changes into guarding the priestess from the other guardians. This is the wine from which the heart of darkness drinks. It is the opportunity to make the guardians of the crystal weak and susceptible to attack. The darkness craves the power of the crystal and also the girl as a man would crave a woman. Hence the entrance into another world where darkness is dominant within the world of light.