Hollow; the hunger.

Synopsis;

Hollow are the souls of humans, originally. Though after death, if they don't pass on, they are over-whelmed with sorrow and anguish. In time, their very souls become contaminated and a metamorphosis takes place; turning what was once a normal soul into a monstrosity which devours other souls. The process normally takes months or years as the Chain of Destiny slowly corrodes. However, the process may be accelerated under certain conditions; such as a soul being consumed by a Hollow, and the previously human soul is bound to its aggressor as subservient.

The Hollow have two key trademarks: the first being a white, bone-like mask which conceals their face (and the identity of the human they used to be), and a hole through their chest cavity which may be the origin of their name. The masks of different hollows can differ greatly in shape and form, but they are always white.

These monsters retain intelligence, though they're driven by their own thirst to fill the hollow feeling they're inflicted with by devouring souls. With each soul a Hollow devours, it becomes stronger; and the stronger the reiatsu a soul possessed, the Hollow's own power is amplified by that amount. Powerful or extremely crafty Hollow have defeated Shinigami and devoured their souls, which greatly advanced the Hollow's on power and in some cases manifested abilities within the Hollow. Any soul devoured by the Hollow becomes a Hollow in turn. Strong Hollows have the ability to command weaker ones, especially if that Hollow is the one who turned the other(s).

Hollow Classification;

Hollow:

Hollow have different abilities and differing monstrous appearances; the low level Hollow are small and can only attack in melee style, where-as the bigger and better Hollow have all sorts of killing implements. Hollow can have an animalistic appearance.

Menos:

Menos are created by numerous Hollow piling up on top of one another and merging as a spiritual fusion; creating an immense monstrosity that generally lacks individuality.

Three classes of Menos have been revealed: Gillian, Adjucha and Vasto Lorde.

Gillian:

As the weakest class of Menos, they are large and physically strong but not agile and don't have much brain power. They are formed when many Hollow merge together. They are much weaker than a Shinigami Captain. In Soul Society, Gillian are known as "Menos Grande," and are shown in textbooks discussing Menos.

Adjucha:

Smaller but smarter than the Gillian, they are thought to be far superior; evolved by devouring other Hollow. It is likely they could fight evenly with at least a Shinigami Lieutenant. It is unknown how powerful they would become as Arrancar and it is implied that some of the Espada are made from this type of Menos. This type of Menos is much smaller in population compared to the Gillian class.

Vasto Lorde:

The most empowered of Menos with an extremely small population and are believed only to exist within the very depths of Hueco Mundo. Each of them is stronger than an average Captain; therefore, Hitsugaya Toshiro claimed that 10 is enough to overcome all of Soul Society.