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 to be grammatically correct Cero Oscura is a black and more powerful rendition of cero that only the Espada are permitted to use. So far only Ulquiorra Cifer if the only one shown to use this variation.
When using this variation, Ulquiorra uses his index finger to compress a vast amount of spiritual energy into one jet-black cero. The result of this being far greater in power than a normal Cero and even exceeds the Gran Rey Cero. It has been noted as "The Espada's fully powered Cero". When it is fired it can turn any exposed light to complete darkness. The range of this Cero is more vast than any other known Cero and is so powerful it's capable of obliterating Ichigo's Hollow Mask - Something that the Gran Rey Cero (Used by Grimmjow Jaegerjaques) couldn't do. This in turn critically injures Ichigo and destroys a large proportion of the roof of Las Noches.
During the fight between Ichigo and Ulquiorra, Ulquiorra referred to it as "Our Cero". He demonstrated it's power in order for Ichigo to realise that his Getsuga Tenshō is similar to a Hollows Cero.. At close range it was capable of blasting a large hole in Ichigo's chest.