What is the Magus Stone in the novel, Illusion: Chronicles of Nick?

The Magus Stone is a piece of the sun that was used as a weapon in an ancient battle between primal gods, which turned into a stone when it hit the desert. The stone was set into a sun medallion and given as a peace offering after the war was over. In theory, this medallion is the only object that can reunite Nick’s soul and body, though no one knows where to find it.