Considered "The Dark Souls of Paper Mario", Master Quest is incredibly difficult.
It was designed with elite challenge runners in mind, and unless you have extensive knowledge of Paper Mario, you may struggle with much of the content.
By Brotenko, Emperor Thamz, Quackles, Rainchus, snakemouth_den, ShyBlookster, and Vinnie64.
Its source code is at https://github.com/Brotenko/PMMasterQuest.