This is the realm beyond legend rank. In order to reach this realm, normally a legend rank expert would cultivate the Power of Heaven. However, in the Divine Continent Domain the only way for a person to reach this realm is through cultivating a law to become a Spiritual God.
Demon Beasts are also able to break into this realm by becoming spiritual gods. It was when a demon beast moved into the Heavenly Fate Realm that ultimately lead to the demon riot and the Age of Darkness.
As a cultivator reaches this realm, they form something called a "Fate Soul" in their soul realm. If they died, they would only lose one of these Fate Soul instead of facing a true death. Ming Fei recently lost three of these other "lives" when fighting against the Demon God Sect in the Draconic Ruins Realm.
For an Earth Fate Realm to reach the Heavenly Fate Realm, one must harmonize with Heavenly Energy by sensing the Heavenly Dao(way).
In the previous life Nie Li was able to cultivate beyond legend rank despite being in the Divine Continent realm and not becoming a spiritual god. He accomplished this by using his own method through the Temporal Demon Spirit Book.