RWBY: WORLD OF REMNANT
The Creatures of Grimm; a ravenous, destructive force that covers the majority of Remnant. While often referred to simply as "Grimm" in the common vernacular, these beings serve as the greatest foe to mankind. For as long as Humanity can recall walking the surface of Remnant, so do they remember this wicked force.
Many ancient cultures believed the creatures of Grimm to be animals possessed by evil spirits, or perhaps the spirits of tortured animals themselves. However, further study -- as well as the discovery of newer, more horrific forms of Grimm -- does not support this hypothesis. With new creatures discovered every day, scientists perpetually find themselves with more questions than answers.
While very little is known of their origin, some key facts have been observed in the wild. First and foremost, the Grimm exclusively attack humans and their creations. While occasional skirmishes between wildlife and Grimm have occurred, these instances appear to be based on territorial provocations, rather than a need for sustenance. In fact, the common belief is that the Grimm are not obligated to feed; they choose to.
What is perhaps even more unsettling is the basis of their attraction. The Creatures of Grimm are lured towards negative emotion. In the rare instances when villages are capable of defending against a wave of Grimm, their survival is not guaranteed, as widespread panic will only lead to more attacks.
Little else is known about the Creatures of Grimm. Keeping them in captivity has proved to be an understandably difficult task, as the creatures tend to either die, or kill those who imprisoned them in the first place.
To further complicate study, the corpse of a Grimm will only remain for a short period of time, before completely evaporating. Those who hunt the beasts for sport find this particularly upsetting, but manage to get by with cheap taxidermic recreations and bombastic storytelling.
Although the Creatures of Grimm appear mindless, more delicate observation has proven contradictory. It has been noted that while younger Grimm tend to be more reckless; older Grimm, who have managed to survive their battles, have the tendency to learn from their experience and will exercise caution in the future. This perverse form of self-preservation allows the creatures to become more effective killers.
And in the end... killing is all that matters.