Unlike the sun that evolved from the Golden Crow in Ancient Times, the sun after Ancient Times is the representation of the Power of the Supreme Yang, possessing boundless might. Countless cultivators rely on the sun for their practice, otherwise, the Yaoyang Sect wouldn't have emerged.
During the Great Qian Period, the sun was sealed by the Mid-heaven Emperor Lord, the founding Emperor Qian, and claimed it as his own.
In the Great Yu Era, the founding Emperor Yu, Wu Yao, saw that the Mid-heaven Emperor Lord had sealed the sun, so he placed another seal on top of that one.
As a result, neither the Mid-heaven Emperor Lord nor Wu Yao could obtain the sun.
As Wu Yao was suspected of being replaced and sent to the outside world, Lu Yang originally thought that the second seal could never be unlocked again. However, Guan Shanhai possessed the method to unlock the second seal.