Caja Negación

is an item given to the Espada by Sōsuke Aizen.

Overview
Caja Negación is a special cube-shaped device held by Espada meant to simulate Negación fields, which are used to punish their Fracción, if necessary. The cube, when used on a subject, traps them in an alternate dimension from which there is no escape; however, this assumes it's being used as intended. More powerful Arrancar, namely the Espada themselves, can escape, given enough time. The only occurrence of this Negación field being used was when Grimmjow Jaegerjaquez trapped Ulquiorra Cifer with it. Apparently, the small cube device must be placed in the Arrancar's Hollow hole, as during their battle, Grimmjow waited to use it until he could place it in Ulquiorra's chest.

Trivia

 * It is called the in the English dub.