The City of Trials was created knowing that the Undying and the Eternal Darkness would be born in less than a Century before the danger of the Veilures would increase tremendously.
The Origin had gone into hibernation after using up a tremendous amount of its energy to construct the City of Trials to twist time to a sheer impossible degree.
Only one living being was able to reside inside the City of Trials at a time, and only the chosen ones were able to stay inside the City of Trials for longer than 365 days.
A year was exactly what it took for one day to pass in the outside world. This knowledge was something Dilan got to know from the Old Librarian, who was more than willing to tell him every single secret of the City of Trials.
Entering the City of Trials meant that one would disappear from the Universe. This disappearance would change the fate of the entire Universe as the domino effect would slowly tear everything down.