From Bleach Wiki
Aizen in Bleach
Aizen Sōsuke is the former captain of the 5th Division and the main antagonist of the series. After betraying Soul Society, he went to Hueco Mundo with Ichimaru Gin and Tōsen Kaname. There, he gave rise to the Arrancar army.
Aizen on Legend of Bleach
In order to advance to the next rank, you must seek out Aizen and defeat him. This is not a fight you should rush into, so make sure you are prepared.