After traveling through Soi-Fong, he was infected with the Hollow Poison. If he didn't use Bankai, he would die.

But:

[Mission: Become the captain]

[Restriction: Until you become the captain, you can't use Bankai (Note: Forcing Bankai will fail and cause explosion)]

[Rewards: 1. Recast the Zanpakutō "Snake Bee" and adjust its ability; 2. Master Bankai. ]

Soi-Fong: I want to be the captain.

Yamamoto: You have to learn Bankai first.

Soi-Fong: I won't be able to use Bankai until I become the captain!

Yamamoto: Then learn Bankai first and then become the captain!

Soi-Fong (throwing a chair and roaring): I already said I can't use Bankai until I become the captain!

Yamamoto (sticking his head out and cursing): Then go! Learn! Damn! Bankai!!

Soi-Fong: Yamamoto! Fuck you Sage!!

Soi-Fong: No, I have to think of a way, otherwise I'm going to die.

The next day.

Yamamoto: This is a captain's meeting, what are you doing here?

Soi-Fong: Captain of the 11th Division, Kenpachi the Hornet! A dignified person!