Sixty thousand years ago.
After Yao had helped the Ancestor of the Nether King to rule over the Netherworld, he never expected that the Ancestor of the Nether King would order his capture.
However, in the end, the Ancestor of the Nether King, remembering their past relationship, did not kill Yao, but imprisoned him beneath the walls of the Netherworld's gate.
Sun Shouyi's grandfather was the person dispatched by the Ancestor of the Nether King to handle this matter! He used a technique to freeze the power within Yao's body, and with the shell of a millennium Nether Turtle, sealed and suppressed the prison holding Yao.
Before dying, Sun Shouyi's grandfather told Sun Shouyi's father how to break the Nether Turtle's shell seal and awaken the power within Yao's blood, a method that was later passed down to Sun Shouyi by his father.