Mortal Emperor was born during the Desolate Expansion Era in the Tyrant's Empire. He was an extremelly talented and powerful cultivator, eligible to carry the Heaven's Will. Alas, he had given up this opportunity for some reason.
Fighting the Tyrant
Instead he became a protector and champion of the Human Race that was heavily oppressed by Tyrant. Mortal Emperor led a rebellion against Tyrant and fought him countless times for many years. However, whenever Tyrant was in danger, he would borrow the power of the secret under his land to defeat the Mortal Emperor.
Eventually, Mortal Emperor became despaired after countless defeats. As a last resort he sent his own daughter, Su Yu He, to marry Tyrant in order to learn his secret. Su Yu He became Tyrant's toy and in his hands she accepted countless degradations and suffered from countless shameless acts, but in the end, she was able to obtain the secret and Mortal Emperor defeated Tyrant.
Mortal Emperor became obsessed with the secret of this land and tried to use its power to forcibly connect and carry the Heaven's Will. However, in the most crucial moment, the hateful sentiment of Su Yu He appeared and attacked him. Due to his guilt, Mortal Emperor's Dao Heart experienced a great blow and he died under the power of the Heaven's Wrath.
Alas, Mortal Emperor was too obsesed with the Secret, so after death his obsession gave birth to a yearning sentiment. His sentiment, powered and twisted by the Secret, had turned into a resentful monster. The Secret's power allowed him to exist for millions of years.
Millions of years later Dark Crow led Min Ren to the is place to the ruins of Mortal Emperor's Kingdom to establish the Cleansing Incense Ancient Sect. Dark Crow wanted to obtain the Secret, but eventually gave up because of its evil nature. He and Min Ren then sealed the Secret and placed the Emperor Foundation above it. They also discovered Mortal Emperor's sentiment. Our of respect for Mortal Emperor's protection of the Human Race during his life, Dark Crow decided to not annihilate his sentiment and instead sealed him together with the Secret. Dark Crow hoped that one day, Mortal Emperor's sentiment would let go of his obsession and dissipate. Alas, it never happened.
At the end of the Emperors Era, Mu Shaodi opened Min Ren's seal to obtain the Secret's power. Alas, instead of power he found Mortal Emperor's sentiment who tried to escape from the seal and almost destroyed the Cleansing Incense Ancient Sect in the process. Mu Shaodi and Sect's Elders were barely able to force Mortal Emperor back. Alas, they couldn't recreate Min Ren's seal, so they used two of Min Ren's Life Treasures and the Benevolent Saber to suppress Mortal Emperor. Even so, the Elders had to stand guard there and use their power to constantly support the suppressing formation for the next 30,000 years.
Li Qiye came to the seal and tried to persuade Mortal Emperor's sentiment to leave the world. Mortal Emperor was unwilling to listen, so Li Qiye took out the Apricot Vase - Su Yu He's favorite vase that Mortal Emperor personally gifted to her when she married Tyrant. The Apricot Vase didn't have any power, but it reminded him about his guilt towards Su Yu He, it represented Mortal Emperor's inner demon. Li Qiye left the Apricot Vase to supress Mortal Emperor, since he can't fight his inner demon unless he lets go of everything. However, once he lets go of everything, he will finally dissipate and leave the world.