Aizen is an extremely powerful combatant, well above many captain-level Soul Reapers. When preparing to activate the Hōgyoku, he states that despite its half-awakened state, it can be fully activated temporarily when fusing with someone with at least twice as much spirit pressure as an average captain-level Soul Reaper, referring to himself. After Aizen embedded the Hōgyoku into his body, he undergoes a series of transformations that give him extraordinary boosts in both speed and strength.
Originally, Aizen carried a zanpakutou called Kyōka Suigetsu (鏡花水月, literally "Mirror Flower Water Moon") that can controls the senses to the create convincing illusions that fool even the most perceptive people. But Kyōka Suigetsu's Shikai ability Kanzen Saimin (完全催眠, literally "Complete Hypnosis"), activated with the command "shatter" (砕けろ kudakero), is only effective if the target watches the Shikai sequence once and thus can fall under Aizen's deception any time he wishes. There are only a few means to escape the sword's influence such as touching the blade the moment it is activated as Gin overheard. Though unaware of that fact, in his gambit to take down Aizen, Yamamoto took advantage of grabbing the blade when it pierced him to counter the illusions. But after his final evolution by the Hōgyoku, Aizen no longer needs Kyōka Suigetsu itself as the weapon completely dissolves with its powers now a part of Aizen's being after returning to his human state.
Aizen captain form
Aizen second form
Hollow form
