Hōgyoku

The is a small orb that dissolves the boundary between a Shinigami and a Hollow. It can allow a Shinigami to use Hollow powers and vice-versa. As Sōsuke Aizen reveals to Orihime Inoue, continued use of the orb while it is in a dormant state seems to weaken it. This item lay dormant in the untraceable faux body that Rukia was in during the Soul Society arc of the manga and anime. The hōgyoku was accidentaly created by Kisuke Urahara,whom was terminated from the Soul Society for creating it.