When Dark Crow obtained the Revolving Crescent Sun Merit Law, he tried to perfect it but couldn't. Through the ages Dark Crow passed it to many of his disciples, including Immortal Emperor Xue Xi, Immortal Emperor Min Ren, who left it in his sect, and Immortal Emperor Tun Ri, but none of them were able to perfectly utilize it. Only at the end of the Emperors Era Dark Crow had perfected this technique.
In the Current Era the Revolving Crescent Sun Merit Law is a notorious technique in the Cleansing Incense Ancient Sect. When Li Qiye chose it in the Scripture Library, Qu Daoli and Library's protector advised him to not cultivate it, but he insisted on his choice.
After returning to the Lonely Peak, Li Qiye studied the Revolving Crescent Sun Merit Law and regained his memories about its profundity. He then used it to purify his Blood Energy into his first drop of the Longevity Blood, thus completing the Palace Foundation level.
Several months later, when Li Qiye reached the Physique Accumulation level, he used the Revolving Crescent Sun Merit Law among other techniques to subdue the Yin Yang Sea of Blood and refine it into his Longevity Treasure.
The Revolving Crescent Sun Merit Law is a Life Wheel Merit Law that use one's Blood Energy to speed up one's cultivation. Unfortunately, it is flawed, so at some point Blood Energy dries out and the cultivation halts. Even cultivator with Saint Life Wheel don't have enough Blood Energy to cultivate it successfully.