Hōgyoku

This item 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.