The one fate that every soul in Ulquiorra is one of Bleach's Arrancar, a type of Hollow that achieved a human-like form and abilities on par with a Soul Reaper. That might not sound so bad, but few Hollows actually become Arrancar, and for some, it's just impossible. But Ulquiorra wasn't always an Arrancar; at one point, he was a low-level Hollow like all the rest. A special manga chapter from a Bleach databook never released in English reveals what his life was like then, and the answer is existentially terrifying.
In the obscure bonus chapter, known simply as "Unmasked" (the title of the databook), Ulquiorra's origin as one of Bleach's low-level Hollow are documented. The chapter has a lot of poetic flair to it, almost exclusively narrated from Ulquiorra's perspective. He recounts his arising as a Hollow, emerging from a pit of absolute darkness, with his first sight being other Hollows ravenously devouring something. Ulquiorra feels different from the other Hollows, however, and wanders the desert of Hueco Mundo.
Bleach's Hollow Afterlife
Throughout his journey, Ulquiorra states that he's unable to hear, touch, or smell, only absorbing the world through his empty eyes. The things he sees barely , and he seeks only to return to the void of nonexistence. He stumbles upon a transparent object that looks like a mass of crystaline branches, which somehow reminds him of the void. Upon entering it, his Hollow mask is fractured, and he feels himself beginning to dissolve away, welcoming the peace. Then he's brought out of his reverie by Orihime, setting all that as a half-ed flashback from a moment during the Hueco Mundo arc.
The existence of a low-level Hollow, at least as Ulquiorra experienced it, is probably one of the most nightmarish fates that could befall a soul. He has no memories of his life as a human and effectively no ego; his senses are almost completely useless; and his existence is utterly aimless, aside from a longing for annihilation. Since most low-level Hollows are just trapped that way with no hope of improving, and seem able to exist eternally even without feeding on souls, there's really no way out of that nightmare without getting slaughtered by a Soul Reaper. It really does explain a lot about Ulquiorra's melancholy demeanor, though, to know that his first experiences as a Hollow were so terrifying, and dominated by a desperate desire to return to nothing. In a way, Ichigo might've done him a favor by defeating him.
Obviously not every Arrancar seems to feel this way, but the chapter is an important reminder of why the work that the Soul Society does is so valuable. People who become Hollows fall out of Bleach's reincarnation cycle, and preventing that is certainly important, but it also helps to save people from the truly hellish fate that befell Ulquiorra.
