User blog comment:Xilinoc/Ch. 586 - The Headless Star 5/@comment-11506376-20140625171259/@comment-11293956-20140627174335

Äs could have just been a survivor of a non Quincy related war that may as well have happened before the series started. Even though we don't know when Yhwach recruited Äs and the rest of the Sternritter, my guess is that they've been around way before the Auswahlen. Since they can steal Bankai, the Sternritter have to have been around long enough to fully understand them.