Board Thread:General Discussion/@comment-1351982-20130515183016/@comment-1387259-20130515205402

I have a feeling that Yamamoto actually wasn't able to kill Yhwach. It's not impossible for someone to be stronger than him, despite that he was the most powerful Shinigami. Not only that, but 1000 years ago, the Gotei 13 was very new.

Anyway, it may not be canon, but look at Arturo Plateado, He wasn't able to be killed, so he was sealed, as well.