User blog comment:Relikz/Chapter 364: Tartarus Arc, Part 1: Immorality and Sinners, Review./@comment-7505640-20131220125754/@comment-1359322-20131220142523

The subject in this case probably means experimental subjects. Kyouka is researching on how to make Humans into Demons after all. Doriate was a failure, Minerva is currently in the process, Erza and Mira are probably next if they get caught for real. Maybe Elfman too if he "gives up" his soul to Sayla.