When Ichigo achieves ban kai for the first time it is, as with many recent events in his life, unexpected. He's fighting Zangetsu as hard as he can, not even stopping to wipe the mixed blood and sweat out of his eyes. He can't afford even the smallest delay. Rukia's executioners will not wait. His spirit sense has expanded so far that he can feel everything around him—he can feel Yoruichi's growing anxiety. Rukia's life, slipping through his fingers with every sword that shatters. He can feel Zangetsu—and then all he feels is light, and the world around him slows down.
The blinding light lasts instant. It takes Ichigo longer to realize it came from him. He looks down.
He can't stop to be proud of himself. Before he can blink, Yoruichi is by his side, thrusting a cape and a seal into his hands. "Take these. Go." He doesn't ask questions. He can feel the soukyoku like it's burning a hole inside him. He takes his teacher's gifts, hoping her speed is one of them, and goes.
Ichigo thinks he should feel different, but the only difference is how himself he feels. He's been off-balance since he recovered his own power. Always on the move, always struggling to catch up. Now he and Zangetsu are perfectly in tune. He understands exactly what he needs to do. How could he not? Things are so clear.
He stops thinking about it. Stops thinking about anything but getting to Rukia. There's so little time.
But as fast as he is, Ichigo has limits, and they can only be pushed so far. And when he reaches the execution grounds, the soukyoku is already descending towards Rukia, whose expression is peaceful and composed. He thinks: No. He thinks: Too slow, I can't be, I can't and her name bursts from his throat in a mighty roar.
Rukia's eyes fly open and the serenity disappears. There is shock—anger and fear in her expression. It is the last thing he sees before the flaming bird descends on her like a funeral pyre.
Rukia screams. It is a short scream.
The flames recede and Ichigo flies to where she stood, but there is nothing left—just a small, shining orb that falling towards the ground. He catches it in his hands.
It is done. Rukia is nothingness. He has failed.
He turns toward the wide-eyed cadre of shinigami behind him. He opens his mouth to speak, to rage at them, but a howl erupts instead, tearing his throat raw.
Ichigo does not resist when the mask materializes on his face.
By dawn, seireitei is ash.