In history, there were originally three Celestial Venerables.
Or at least, there were three.
Because Celestial Venerables are not immortal either; when they are nearing the end of their lifespan, they can only rely on Undying Substance to extend their life. Otherwise, the previous two Immortal Kings who emerged from the muddy path and suddenly died serve as examples.
They lived for millions of years, which far exceeded the limit of an Immortal King. So, once they lose the support of the Undying Substance, they will instantly die of old age.
The advent of Undying Substance began in the era of Celestial Venerable Wuchen.
So, perhaps there were Celestial Venerables before, but they have all transformed into Dao in the endless years.
Shi Hao battles Celestial Venerable Wuchen.
