Spirit Immortals encounter Thunder Tribulation every five hundred years. If they endure through the Nine-Tiered Thunder Tribulation, their Divine Soul is tempered, giving them a chance to advance to become Human Immortals.
Human Immortals undergo the same, facing Thunder Tribulation every five hundred years. Successfully passing through the Nine-Tiered Thunder Tribulation offers a chance to advance to Earth Immortal.
And so on, until the Celestial Immortal crosses the Nine-Tiered Thunder Tribulation, they have the opportunity to condense the Primordial Dao Fruit, becoming a Daluo Golden Immortal, sharing the longevity with the heavens and brilliance with the sun.
"Crossing a single tier of Thunder Tribulation can temper the Divine Soul and greatly enhance mana…"
Wu Liu rambled on at length, although he was not familiar with the cultivation associated with crossing tribulations; what he knew was also hearsay.
"Once every five hundred years, there's a Thunder Tribulation?"
