The Fundamental Virulence

Slipping into Mayuri's hidden laboratory by way of the Dangai, Urahara sets down a cloth of small black pills he calls Shin'eiyaku and asks for the captain's help. With Akon linking the enemy's own machinery to the room, Urahara addresses every surviving captain and lieutenant through a Tenteikura spell, explaining that each pill dissolves into the soul of any Bankai-wielder who touches it.

He traces the discovery to Hueco Mundo, where he watched Quilge Opie lose command of himself after absorbing Ayon. Since an Arrancar's Resurreccion so closely mirrors a Bankai, the Quincy should have been able to steal those too, yet not one was ever taken despite their willingness to conscript Arrancar. Mayuri reasons that stealing a Resurreccion must carry some hidden cost.

Together they lay it bare: Quincy possess no defense against Hollows, so Hollow power festers within them, decaying their souls and barring them from ever Hollowfying. That vulnerability is the true reason they are driven to hunt Hollows at all. If a Shinigami briefly draws a sliver of Hollow power into a Bankai, that Bankai turns to venom for any Quincy who holds it. As Hitsugaya's fingers close on a Shin'eiyaku, Cang Du's stolen ice wing rips from his back in a spray of blood.

Urahara broadcasts his Shin'eiyaku pills to every captain and lieutenant through a Tenteikura spell. He and Mayuri deduce that Hollows are fundamentally poisonous to Quincy. The pills render a reclaimed Bankai toxic to its thief, first seen as Cang Du's stolen wing breaks off.

The Fundamental Virulence is a chapter of the sixty-second volume, HEART OF WOLF, in the Thousand-Year Blood War. It was adapted as Episode 382, which carries the same title. The chapter's own wiki page is a disambiguation notice, so this account is based on the matching Fandom coverage of its events.