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After breakfast, Noah decided to go to the underground area within Arthur's family territory, where Arthur had said that his grandfather had sealed many of the things he owned within a magic door.
This magic door was impossible to access by his parents and even Arthur, and it had been kept in here since his grandfather died.
Arthur had implied that his grandfather had special abilities even before the Apocalypse, according to his father, the son of his grandfather.
His grandfather, although gentle and a good person, was always mysterious in what he did.
However, it was not after the Apocalypse that he revealed part of his powers, which he used to protect his son and his wife, alongside the recently born Arthur a few years after the Apocalypse started.
Check "The Protagnist's Sister Is Actually the Strongest"!