There are very few legends that cross over cultures and times the way the crystal skull legends do. They are contemporarily shared by indigenous people all around the world. These legends have been handed down from generation to generation for thousands of years, which attests to their enduring power. It is said each skull contains '13 spirits of power', imprisoned into the skull even before any human or mutant existed.
Crystal skulls give the user mastery over Time itself, if the spirits are sated with time taken away from others: 50% of damage dealt while wielding it will be returned as Time Units (Mind effect). The Skull can be also used to read minds. The price for that power is heavy, as usage might lead to insanity or even death.