Mutants are essentially creatures with the genetic traits of two different animals. Usually, the mutations are caused by interaction with Tungay, a yellowish slime that Duchess Rebecca discovered while she was exploring an archaeological site. She was into excavation and artifacts, but not until she ended up obsessing over Tungay, who created different types of strong mutants. With multiple trials and errors, the current Tungay can turn human with additional animal traits and superhuman strength or mana skills.
To learn more about tungay, She needs humans to be experimented on. That is why she asked the king, her brother and lover, to give her the prisoners as her test subjects. Of course, the king won't publicly allow her to have the prisoners. The king has a kind and dignified image towards his subjects, and agreeing to Rebecca's request is something against his image and the law that he created.
