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Cero Oscuras (黒虚閃 (セロ・オスキュラス), sero osukyurasu; ; ; Spanish for "Dark Zero," Japanese for "Black Hollow Flash") or more grammatically correct Cero Oscuro, is a full-powered black Cero that an Espada can use only in their release forms. Only Ulquiorra Cifer and Lucio César has been shown to use this technique so far.
When using this technique, the user concentrates his vast spiritual energy with only his index finger into an condensed jet-black Cero with great might. The result of being used in his released state is a much greater Cero, exceeding even Gran Rey Cero, the ultimate Cero, and has been referred to as "an Espada's fully-powered Cero." When fired, it covers any exposed light into absolute pitch darkness. The Cero Oscuras's range is vast, and is mighty enough to completely shatter Ichigo Kurosaki's Hollow mask (something Gran Rey Cero couldn't do), critically injuring him in the process, and strong enough to destroy half of the dome over Las Noches.
Ulquiorra referred to it as "our Cero." Ulquiorra demonstrated the attack while in his release form to prove to Ichigo that his black Getsuga Tenshō is similar to a Hollow's Cero. It is strong enough to blow a round hole into Ichigo's chest. Each user's version has a colored outline that is different from user to user.