In Tite Kubo's Bleach, the monsters that roam the night are Hollows, evil spirits of the dead that devour the souls of humans. And sometimes, each other, too.
Most Hollows are individuals and have limited combat power compared to Soul Reaper officers, but if enough cannibalistic Hollows meld together, they form a giant known as a Gillian, the weakest of the three classes of Menos Grande. At that point, obsessive, cannibalistic hunger is all that defines these creatures. They would eat the whole world if they could.
Amassing Strength
The very concept of a Hollow is disturbing enough. When a person dies, their soul will linger on the mortal plane if they have unfinished business or unresolved feelings. When the chain binding them to the mortal plane is broken, a hole is torn open in their chest, and they mutate into a monster with a bony skull mask: a Hollow. Ordinarily, these spirits are only strong enough to prey on human souls and low-level Soul Reapers, but a few Hollows need more. They are so ravenous, they will turn on their own kind, and a number of such Hollows may meld together if they convene in mutual hunger.
This process is rarely, if ever, seen in the franchise, but it is well-documented all the same. A gaggle of cannibalistic Hollows will devour each other and meld to form a single giant from their collective power, the standard Gillian-class Menos Grande. Such beings all look the same, with a tall, black-robed body and a skull mask with a long nose. Each one has a hive mind composed of every Hollow that's inside, meaning they are mindless creatures. For some Gillians, that is the end of the road, and they shamble around looking for victims to destroy. Many ordinary Hollows can talk, but these cannot, and they operate entirely on instinct. But, for a select few Gillians, their obsessive hunger is still not sated. That's when true Hollow evolution begins.
Some Gillians contain a single Hollow inside that is so ravenous, so strong-willed that it will commandeer the entire creature. When that happens, the Gillian mutates to assume a new, unique form that represents the chief Hollow inside. That Hollow retains the powers of the others, but operates as a single entity. Many such Gillians roam the white sands of Hueco Mundo, and they sometimes form small packs for the sake of survival. Grimmjow, in his Hollow days, met such a group, led by Shawlong Kufang. Other members include Edorad Leones and Ilfort Grantz, but Grimmjow outclassed them all, even when combined.
That obsessive hunger is now the #1 priority for the single-minded, mutated Gillian, and a certain fear takes over as well. It takes a lot of energy for one Hollow to dominate its own body, and if it slips, then it will fade back into the mental cluster inside the Gillian and be lost for good. It's a tenuous position, driving Gillians to ruthlessly hunt other Hollows (including other Gillians) to maintain their strength and mastery over themselves. That desperation will drive them to great lengths, even challenging enemies that clearly outclass them. D.Roy, a rather weak Gillian, tried to eat Grimmjow, an Adjuchas-class, despite the serious difference in their power levels. An Adjuchas is the next evolution of Menos, having evolved from a Gillian, but an Ajduchas suffers the same desperate hunger as Gillians. If left hungry for too long, an Adjuchas will degrade back into a mindless Gillian state, resulting in a life of constant fear.
In short, a cannibalistic Hollow will either lose its mind in a shambling giant body, or it will gain dominance but face the constant threat of devolving back into a aimless hive-mind. The hunger never ceases, and neither does the struggle of a Hollow's cursed life, making them dangerous to their prey.