Hōgyoku

The orb created by Urahara (and, independently, by Aizen) that dissolves the boundary between Soul Reaper and Hollow and grants the wishes of those around it. Hidden in Rukia's soul, it is the missing piece of Aizen's plan.

Fused into Aizen's chest, it makes him evolve beyond every limit; but when Ichigo surpasses him with the Mugetsu, the Hōgyoku 'decides' to abandon him, allowing Urahara's seal to activate.