Board Thread:General Discussion/@comment-3423279-20160110184118/@comment-3423279-20160110200457

Well, my theory on that is that the Shinigami had something before Zanpakuto were invented that could purify souls but didn't have a will of their own, a Proto Zanpakuto of some sort. Nimaiya created the Asauchi at some point as a better or more powerful weapon for the Shinigami to use.