In the Bleach franchise, the monsters of the night are Hollows, the anguished and starving souls of the restless dead. And there's more than one breed of them.
Ordinary Hollows are derived from single human souls. When a person dies with attachments to the mortal realm, their soul is bound by a chain. Once the chain is broken, a hole emerges in their chest, and they mutate into a beastly Hollow. Often, that's the epitome of the Hollow form, but they can advance far beyond that, and the Vasto Lordes, a breed seldom seen in the anime series, are the ultimate Hollows. Even Soul Reaper Captains fear them.
Bleach's Vasto Lordes Are Monstrously Powerful
Hollows are eternally hungry, desperate to fill the hollow hole in their bodies (a futile quest). Their main food source is human souls (of the living and dead alike), but some Hollows are so ravenous, they turn cannibal. If enough cannibal Hollows meld together in mutual hunger, they will actually fuse to create a hive-minded giant: a Gillian-class Menos Grande. These creatures are uniform in appearance and operate on instinct, but if one Hollow inside has a strong will, it can assert control over the whole and regain its independence. The entire Gillian will mutate again and assume a unique shape, and from there, it must keep eating other Hollows to maintain the strength necessary to stay unique. If not, the Gillian will revert to its original state, and these unique Gillians are desperate to avoid falling back into the mindless horde inside.
The strongest Gillians can mutate into an Adjuchas, a smaller, more powerful Hollow that must continue to devour other Hollows at all times. But as Grimmjow and his Gillian allies found out, Menos Grandes cannot evolve beyond that point if they do not have the seed of a Vasto Lorde inside. In short, a Vasto Lorde is destined to be, and the likes of D.Roy and Shawlong Kufang cannot become Vasto Lordes through hunger and willpower alone. Gillians and Aduchas are made up of many Hollows compressed into one body, but a Vasto Lorde is truly a one-Hollow army of incomprehensible power. Only a few exist, and not even the Soul Reaper Captains can defeat them.
There Are Many Vasto Lordes Throughout Bleach's Story
By now, a small handful of confirmed Vasto Lordes exist in the lore of Bleach, though some of them were not confirmed as Vasto Lordes until years after certain chapters of the manga were published. It stands to reason that the top Espada are Vasto Lordes, and indeed, some of them are. Tier Harribel, the third Espada, was among the first confirmed Vasto Lorde. As a Hollow wandering Hueco Mundo, she had a small, humanoid body, the trademark of a Vasto Lorde (setting them apart from the beast-like Gillians and Adjuchas). Harriibel wasn't alone, either; she adopted Mila Rose, Apacci and Sung-Sun as sisters of sorts, banding together for mutual protection. This friendship continued into their Arrancar days.
Barragan Luisenbarn, the second Espada, was recently confirmed as another Vasto Lorde; most likely the chronologically first, in fact. He is an ancient Hollow who was born with the potential for ultimate evolution, and after devouring enough Hollows around him, he ascended to Vasto Lorde state. With that enormous power and experience, he united Hueco Mundo into a planet-wide kingdom of Hollows, and he ruled uncontested until Captain Sosuke Aizen arrived to challenge him and form an Arrancar army.
Ulquiorra Cifer is another Vasto Lorde, and in fact, an unnamed Vasto Lorde resembling him was used as a visual aid while Captain Hitsugaya explained the nature of these Hollows to Ichigo. Ulquiorra was part of a small tribe of black-bodied Hollows who had bat wings and horns, and he was cast out for having a white body. Eventually, Ulquiorra mutated and became an Arrancar, and his second release form featured a black body. Thus, he was accepted back into his tribe, though he hid that form from his master, Sosuke Aizen.
Lastly, Szayelaporro Granz was once a Vasto Lorde, but he intentionally devolved back into an Adjuchas so he could evolve into a "perfect being" via his own designs. The castoff part became his brother, Ilford Grantz.