In my opinion the challenge is probably solving the problems that may corrupt the people that are on the list and prevent them from ever committing a crime. Since its Yama's job to be the judge of the afterlife after all. And from the word Future Sinners, it meant that it still hasn't happen and it can be changed.
Am I the only one who didnt expect this manga's hardcore setting?
i was expecting it to be cute and fluffy, but nope!