Bankai.
Following the Zanpakuto's first-stage release... Shikai, it was the Zanpakuto's second-stage release, as well as its ultimate form.
It allowed for a more thorough and comprehensive release and use of a Zanpakuto Ability.
Even throughout Soul Society's long history, there had been very few Shinigami capable of reaching this level.
Because it not only required the user to possess extraordinary Spiritual Might.
It also required them to be of one mind with their Zanpakuto, to fully converse with it, or to make it submit.
No matter the method.
No matter the school.
All of it would surge like rivers flowing into the sea, converging at the end into a fixed result.
A new form after releasing one's Zanpakuto, and the Shinigami's ultimate means.
Aizen had already stepped before Tōsen Kaname, his expression now seeming somewhat moved.
"To force a captain-class figure to this extent... Perfect. Is this your true strength, Shizuya-kun?"
BOOM!!! A heaven-shaking hum erupted like thunder. Though it was already deep into the night, a violent gale suddenly swept through the Seireitei.
Crack, crack, crack, crack, crack...
The clear moon overhead had vanished at some unknown point.
In its place hung murky, oppressive storm clouds, as though they would swallow the entire sky.
Spiritual Pressure spilled into the air, filling it with a presence that made it impossible to look away.
That display of might instantly made the two men in the distance sense something unusual.
Cold sweat immediately broke out across Tōsen Kaname's body.
He could not stop trembling. His subconscious screamed madly, issuing a shrill cry.
Instinct told him to get away, not to draw any closer.
Tōsen Kaname unconsciously took half a step back.
But Aizen gently steadied his back and spoke at that moment.
"Kaname, calm down."
As though waking from a dream,