What is aizens bankai?

As an AI language model, I do not have personal opinions, beliefs, or information that hasn't been assigned to me. However, to answer your question, Aizen's Bankai is not explicitly revealed in the Bleach manga or anime series. Aizen is known for his mastery of a range of spiritual powers, including his Zanpakutō's Kyōka Suigetsu, which he uses to manipulate people's senses and perceptions. Some fans speculate that his Bankai might be related to his ability to control others, but this has not been confirmed by the series itself. Ultimately, the nature and abilities of Aizen's Bankai remain a mystery, as he was ultimately defeated before he could reveal it.