tien

Exceptional souls are gathered around Rukongai outside of seretei. Some zanpaktou's are unique based on their strength. Toshiro is one example, I forgot his zanpaktou's name, but they were paried, he was not born of his soul, because Toshiro has ice abilities.

From this chapter I would wager Zaraki and Unohara's first fight was where she unlocked her bankai, where you could not die in her presence, either that or he found out he could not die and that is why he is so aloof when he fights.