Nine Categories (16th Cycle)

Details

Not much is known about the sixteenth regression, except:

Eun-hyun believes it lasted five hundred years.
He remained with Hong Fan.
He only remembered "Gu Ju", which was presumably a name he gave to someone or a name he received.
Hong Fan achieved Entering Heavens Beyond the Path.
According to Kim Young-hoon this cycle must have been more than 20 billion years in reality, as he claims Eun-hyun is older than him, despite spending 20 billion years in the Outer Sea.
From chapter 683, the line "Judging by its depth, the history Seo Eun-hyun experienced in the 16th cycle must have been overwhelmingly immense..." supports this.
Seo Eun-hyun went to the Outer Sea through the Heavenly Void Furnace and made a promise with the Heavenly Venerable of Time.
Most likely ended with his memories sealed by a True Emperor.