The Kingdom Stone is the crystalline concentration of the energy supply from the starship that brought the Ancient Dragons of the Before Time to Avalice. It's used by the Avalicians to power their technology. According to the Royal Magister, the Three Kingdoms of Avalice: Shang Tu, Shang Mu & Shuigang have depended on the Kingdom Stone for centuries.
Before its destruction, the Kingdom Stone resembled a sparkling crystal orb. It had the colors of opal (purple) and sapphire (light blue). After its destruction, it becomes a crystalline vortex of the same colors and has most likely became the planetary ring that surrounds Avalice.
By the time Lord Brevon and his army crashlanded on Avalice, the Kingdom Stone's power was fading, unable to keep up with the growing energy demands. Desperate to avert an energy crisis in Shang Mu, Mayor Zao hires Spade to help his Soldiers steal it from the ancient temple in order to not only preserve his city's power, but also to increase his chances for a re-election. Around that time, Brevon invaded Shuigang, murdered the King and brainwashed his son Dail, the only Heir to the throne, to steal the Stone to recharge his Dreadnought so he could continue his galactic conquest, leaving the Three Kingdoms on the approach of civil war.
After Lilac manages to convince the armies of Shang Tu and Shang Mu to unite against Brevon with the Holodragon, a large assault on the Dreadnought's refueling station takes place, but not before the warship makes liftoff with only Lilac, Carol, Milla, and Torque getting onboard. When Lilac and Carol almost succeed in disconnecting the Stone from the ship's power core, Brevon forces them to stop their attempt when he captures Milla and turns her into a mutant. Milla is defeated and falls into a death-like coma, enraging Lilac and Carol, who break through the last of Brevon's defenses and make their way to the power core of the Dreadnought, where they confront and defeat Brevon. After the batle, the Dreadnought explodes, taking the Kingdom Stone with it. When all hope was lost, the Stone reincarnated itself as a crystalline vortex that hovers over the Avalice, spreading unlimited energy across the planet, marking a new beginning for the people of Avalice.