|Spoiler Alert! This article describes a subject that is sensitive to plot development.|
Fi's Crystal is an orange colored diamond-like crystal that Zelda has to sleep in to maintain the seal on the demon king, Demise. When Zelda and Link find the second Gate of Time, it is revealed that Princess Zelda is the reincarnation of Hylia, the goddess that had taken care of the legendary Triforce ages ago.
Zelda awakes from her slumber after Demise is defeated by Link, but is then taken by Ghirahim.