First ever mentioned in Soldiers At the Heart of the Sun, the first ever appearance of the object was in Thieves of Olympus Mons. The Heart of the Sun is a jewel given to Malona Ren by the Great King Fain as a symbol of love. Rumored to be the power source of "all that is," the jewel (through the royal marriage) was to be the tie that bound the last of the great kingdoms together. Once the two kingdoms fell to war, the jewel was lost. For 1000 years, the jewel existed only in rumor and children's fables. After Bagworm hears of its existence, a plot is hatched to invade the fallen city of Soraian Beth and steal the jewel. It's only after Bigfoot and Bagworm are captured within the lost city they discover Malona Ren is alive and well and still within possession of the Heart of the Sun.
From the Chronicle of Castor:
"For a thousand cycles,
It existed only in rumor.
A child's bauble...
A city's power...
A madman's treasure..It was many things in many tales.
The source of all knowledge,
And the curse of a princess.
It was the jewel that shaped the world.
The Heart of the Sun.
Thousands would die to possess it.
Kingdoms were buried to destroy it.
But no one had ever tried to steal it."