During the Ancient Immortal Era, Pan Gu Universe was filled with many powerful great ones, with a total of seven Primordials at the pinnacle of power. Along with the Daoist Progenitor Hong Jun, that was a time of peace and prosperity for Immortals. However, everything changed when another universe invaded.
That was right.
The invasion of another universe was already present during ancient Immortal times.
The intensity of the battles at that time was intense.
Finally, Hong Jun made a sacrifice by merging his body with Pan Gu Universe's Great Dao. It was due to that newfound power that the Alternate Universe's invasion was pushed back.
Back then, the pathway where the Alternate Universe invaded was known as the Heavenly Rift. That rift was sealed shut later on by Nuwa using the five-colored stone.
That was how the ancient legend of Nuwa using the divine stone to mend the sky came to be.