A magical artifact created by the Sun Goddess, which contains her essence and shades of all the powers she is capable of wielding. It was originally intended as a gift for Malaphar, God of the Shade, but the Dark God attempted to use it for nefarious purposes, so she took it back, resulting in one of many wars between the two of them.
The Heart of the Sun was stolen by Kerse under the orders of Malaphar. But rather than bringing the stolen heart to the upper-god he served, he chose to give it to the human woman he fell in love with, thus starting a sequence of world-changing events that led to the Shadows and Crowns Saga.
In The Call of the Void, it’s revealed that the Heart was formed from a rock on the shores of Dawnskeep, where the blood of the twelve Vitala was spilled. Grieving the loss of these guardians she’d created, Solatis carried the rock with her over the centuries, allowing it to absorb more and more of her power.