We've been waiting until now!
After the Main God unfolds, an area similar to the Dusk Territory will appear, where Zheng Yichen can show up.
He and Tia cannot appear on the 'surface layer' of the New World; once they do, it will trigger the growth of the Old Dusk.
Both Zheng Yichen and Tia are 'deep-rooted' existences of the Old World, but Zheng Yichen is slightly better off because he forcibly joined the Dusk people, and additionally exhausted his Old World time records, so his appearance has a minor impact.
But Tia is different—she is a concentrated singular entity of 'Old Dusk'; appearing on the world surface would directly cause a singularity explosion implanted in the New World from the Old Dusk.
This makes it seem like Zheng Yichen's appearance is not an issue, but to deal with Tia, Zheng Yichen forcibly eroded her using the Ark of Life, establishing a special connection between them.
