You can read Bleach 523 at this online popular manga site. Here is the summary of Bleach chapter 523: Ichigo and Renji find themselves pitted in a test of combat with nameless Zanpakutō, which Nimaiya reveals to be the Asauchi. Nimaiya explains that the Asauchi every member of the Gotei 13 is given upon graduation from the Shin'ō Academy is essentially a blank template, and that over time the essence of the wielder's soul is projected onto the blade, thus creating the wielder's personal Zanpakutō. These Asauchi are all made by Nimaiya himself.
Meanwhile, on the lowest floor of the Central Underground Prison, "Muken," Zaraki Kenpachi and Unohana enter a dark cavernous space. Unohana explains that this is the only place she and Zaraki could freely fight, and that it was only due to special permission from the Head Captain that they were permitted there. Under normal circumstances, no one, even condemned criminals are forbidden to enter, to which Zaraki states that if not for their strength they would be deemed criminals themselves. Unohana gives Kenpachi a chilling look, and remarking upon how talkative he has been, reveals a scar above her heart. Unohana says her wound is throbbing, and Kenpachi echoes her, pointing at the scar over his left eye.
In another room Captain Commander Kyōraku remarks upon the strength of the original Gotei 13, and says that their 11th Division Captain (Unohana) was famed for her mastery of different swords and schools of swordsmanship, taking the name "Yachiru." He then says that when Zaraki and Unohana clash, it is inevitable that one of them will die. Back in Muken, the two jump towards each other and the fight begins.
