The Heaven's Will (天命) is the gathering of the myriad dao. It requires the gestation from the worlds themselves and it contains the power of the primordial chaos, the power of the source, the power of the seven emotions and six desires, and the power of the heaven and earth. The Heaven's Will is influenced by living beings so is slightly different in every generation.
The Heaven's Will serves as a shackle; it encompasses and traps all living beings. Once a cultivator reaches the pinnacle, their goal is to control the Heaven's Will so that they will finally be free from the prison. This was why one will be called Immortal Emperor once they have obtained the Heaven's Will.
In the Nine Worlds only one Heaven's Will can be born in each generation, so only one person in each generation can become an Immortal Emperor. After the formation of the Heaven's Will, all compatible candidates are recognised with a mark in their forehead. When one of the candidates win the competition, the Heaven's Will transforms itself to match its new master Grand Dao.
After shouldering the Heaven's Will Immortal Emperor can open a small crack in the barrier between the Nine Worlds and the Tenth World. The crack needs to be big enough for Immortal Emperor to ascend to the Tenth World.
However, once the Heaven's Will is obtained, Immortal Emperor can't stay in the Nine Worlds for long and should leave to the Tenth World. Any Immortal Emperor who stays in the Nine Worlds would have to face the Heavenly Tribulation from Old Villainous Heavens. That's why when the time was right, all Emperors chose to ascend.
Moreover, if the Heaven's Will continued to stay in the Nine Worlds, no juniors would ever be able to rise and the Nine Worlds would be drained and the myriad races would wither. The Heaven's Will begins its rebirth from the moment the last Immortal Emperor departs to the Tenth World.
Once an Immortal Emperor attains Heaven's Will they fuse it into their Fate Palace. One Fate Palace can only hold one Heaven Will. The Nine Worlds' Heaven Will possess an endless potential and could grow stronger with time. Normally a Nine Worlds' Heaven Will could be as powerful as 6 Tenth World's Heaven's Will, and the weakest is at least equal 2 Heaven's Will.
Heaven's Will Secret Law
When an Immortal Emperor receive the Heaven's Will, they would be able to create their own Heaven's Will Secret Law, a technique that communicated with the Heaven's Will; the strongest cultivation technique apart from the Nine Grand Heavenly Scriptures.
Destruction and Restoration
Without the Heaven's Will, the world's energy was depleted. Cultivators that had reached Enlightened Being level and above couldn't take another half step forward because of the dried up world's energy, so their cultivation remained at the same place.
Ten years before Li Qiye returned to his original body, the Heaven's Will started to slowly restore, which marked the beginning of a new era and a chance for cultivators to become new Immortal Emperor.
In the Tenth World, there are 72 Heaven's Will born from the energy of the 13 continents and more than one person have the chance to obtain 1 or more Heaven's Wills, if after 10 000 years pass, the Heaven's Will dissipate and return to the World to be born again. If some of the 72 Heaven's Will remain unobtained, it will return to the world as primordial energy helping young in their cultivation. 
In the 72 Heaven's Will, there are 4 Primal Will and are a little more difficult to obtain than the other, as well as being the 4 First Heaven's Will to be born.[?]
The Tenth World's cultivators have three chances to absorb (shoulder) Heaven's Will. Each person is capable of infusing four Heaven's Will into their Fate Palaces each time. If a Cultivator was to shoulder four Heaven's Will each time and had 12 Fate Palaces then they can become a 12 Heaven's Will Emperor which are the apex of Cultivators of the Tenth World of which there have only ever been nine. However all Heaven's Wills should be obtained in the same generation, they can't mix Heaven's Wills from different generations.[?]
Upon reaching the Tenth World, a Immortal Emperor could continue strengthening their Nine Worlds' Heaven Will, like Immortal Emperor Jiao Heng who became strong as a 12 Heaven Wills Grand Emperor; or they can try to shoulder additional Heaven's Wills from the Tenth World, like Immortal Emperor Wan Gu who was able to wield 8 more Heaven Wills. For the latter option, one need to change their cultivation to adjust to the Tenth World's Heaven's Will (an error could cause their death) and they only have two chances to them, so eight Heaven's Wills is their limit. However Immortal Emperors don't need to pick their Heaven Wills in the same generation and could wait to the next.