bankai means second (Or final) release.
1. Zanka no Tachi. Captain-Commander of the Gotei 13 and the strongest Shinigami, Yamamoto's Bankai condenses all the flames previously released with his Shikai into his sword. This makes it hotter than the sun and allows it to cut through anything.
Ichigos Bankai is called Tensa Zangetsu and looks like a regular shinigami Zanpakuto apart from the fact that the Bankai is completely black in its incomplete state with a chain at the end of the hilt. Ichigo's final bankai in the TYBW arc is both his hollow second release and soul reaper bankai combined.
His Bankai is one of the most powerful and he almost ended the last Bleach battle too soon. With his Shikai, he is able to mark his opponents with ink. This marks allowed him to change any property of his opponent in his Bankai form, basically allowing him to win the battle with just one word.
Shukai is only reserved for few Shinigami because of its strict requirements. Being the form beyond the Bankai, the first and most noticeable difference between Shukai and Bankai is the power increase.
Aizen probably didn't use Kyoka Suigetsu or its Bankai on Ichigo in their final fight because even if he could, his reiatsu wasn't powerful enough to effect Ichigo, who was a "dimension above".
Kyōka Suigetsu (鏡花水月, Kyouka Suigetsu; literally "Mirror Flower, Water Moon") is an illusion-type Zanpakutō spirit that had once belonged to Sōsuke Aizen.
The Soul King (霊王, Reiō) is the supposed ruler of Soul Society who resides in the Soul King Palace and is protected by the Royal Guard. In reality, he is a god that has been sealed, and his existence is tied to that of Soul Society, the Human World, and even Hueco Mundo. The Soul King is the father of Yhwach.
Upon his defeat by Ichigo Kurosaki, Aizen loses all of the physical changes made by his transformation and returns to the physical form he had prior to transforming. While imprisoned in Muken, he wears an almost entirely black outfit with white trimming, a large black eyepatch over his right eye and black boots.
Originally Answered: Why did Gen try to kill Aizen? Gin tried killing Aizen because he didnt think what Aizen was doing right. He knew and saw how strong Aizen was, so joined forces with him and followed Aizen as a leader and found out everything about him, his weaknesses first of all and how he could get rid of him.
The concluding antagonist of Bleach turns out to be the Quincy originator, Yhwach, who is resurrected 999 years after his previous "defeat" at Yamamoto's hands.
Ulquiorra was always seen as a powerful enemy, but it was in his clash with Ichigo at Hueco Mundo that he released his final form, enhancing all of his capabilities and dominating against Ichigo.
As Ichigo's inner world begins to crumble, Ichigo asks if he is Yhwach, prompting Zangetsu to state he is the manifestation of the Quincy powers within Ichigo, and he both is and is not Yhwach.
In actuality, Old Man Zangetsu is a manifestation of Ichigo's Quincy power, while White Ichigo is the manifestation of his hybrid Shinigami-Hollow powers.
1 The Zero Squad Makes Yhwach The New Soul King.
Nimaiya states that the entity he believes is Zangetsu is actually a form of Yhwach, and that his real zanpakuto is the inner hollow within him, White Ichigo. However, the hero realizes that they are both, in fact, equal parts of his personality as well as his history, accepting them together.
Yhwach was born with the innate ability to distribute a piece of his own soul onto those that would come into contact with him. In doing so, that which people lacked which could not be obtained through the strength of a single soul was finally within grasp.
Yhwach, and Ichigo himself, were already “more powerful" than the Soul King by virtue of their ridiculous skill sets and indomitable stats. (The Soul King used to have abilities that altogether would make one...
Masaki Kurosaki (黒崎 真咲, Kurosaki Masaki) was an Echt Quincy and the wife of Isshin Kurosaki, mother of Ichigo Kurosaki, Karin Kurosaki & Yuzu Kurosaki. She was killed 9 years before the resurrection of Yhwach while trying to protect her son from Grand Fisher.