The portal Amon sent the Soul Flayer into was the same prison dimension that trapped Yuka, the Zuanze Butterfly.
Amon didn't bring his captive to the room where Yuka was imprisoned. If he put the Soul Flayer into the same prison as Yuka, she would devour the Soul Flayer. While the two spirit beasts were both Higher Mythical beasts, Yuka was much stronger. It was the same comparison between a Peak Lord Stage and a Beginning Lord Stage.
Amon brought the Soul Flayer into another room, which had the same prison formation as the one in Yuka's room. A Khode's Formation Sage who served under Amon built this second formation.
Amon copied the formation he observed in Yuka's room and showed it to the formation sage, who studied it diligently for some time. When the formation sage was confident that he could replicate the formation, Amon brought him inside the metal box.
