What is aizen bankai?

There is no official information on Aizen's Bankai as it has never been shown in the anime or manga. Aizen's zanpakuto is called Kyoka Suigetsu, which has the ability to create convincing illusions that can affect any of the five senses. However, it is speculated that Aizen's Bankai would enhance this ability and make his illusions even more powerful. Aizen is known to be one of the most powerful characters in the Bleach universe, so his Bankai, if it were ever to be revealed, would likely be incredibly devastating.