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I keep thinking it's Yammy modified with Szayel Apollo's technology, because Mayuri wanted to dissect him after he took Szayel's stuff.
Szayel could nullify bankais and Quincy powers, so I'm thinking Yammy's infinite rageboost + anti-Quincy and anti-Bankai powers. Fitting because the Sternritter will be using either Quincy powers or the Bankais they stole