Anyone who had used a Grotto Heaven Secret Realm to survive past their lifespan limit would immediately face the Celestial Five Declines upon leaving. Their remaining life would be short, lasting anywhere from a few breaths to several hours.
Therefore, regardless of whether the path to Ascension was severed or not, the fanatical exploration of reincarnation and recultivation had never waned.
Those Great Powers created all sorts of unimaginable Secret Techniques, but not a single one was ever proven to lead to a successful reincarnation. They were all just ethereal rumors.
The Underworld Nether Coffin had such a great reputation because, according to legend, several Innate Divine People had used it to reincarnate and went on to accomplish great deeds.
Whether these tales were true, or if they were just later generations fabricating connections to grant themselves noble past lives, was hard to say.
Most likely, only the Moral Sect possessed detailed records regarding this matter.
