The nine strongest people were above all living beings. Gradually, they were no longer satisfied with being "humans" and began to call themselves "Immortals". The nine of them were the Immortal Ancestors.
Moreover, in order to differentiate the original world they were in from the immeasurable universes and the worlds in the Chaos Sea, the Nine Immortal Ancestors named this world the Primordial World.
Using the once in 5.6 billion years Chaos Sea Tide as a reference, they established a time unit called the "Primeval Era".
From then on, the era ruled by the Nine Immortal Ancestors started.
Moreover, based on the concept of Immortals, the concepts of Immortal Emperors, Immortal Kings, and Immortal Monarchs naturally appeared in the years after.
At first, there was no uniform restriction on the use of these titles. It was relatively chaotic.