Grimm

The Grimm are monsters introduced in the first episode of RWBY, which inhabit various parts of Remnant. They are described as "creatures of destruction" and lack a soul; hence, they are unable to use Aura.

History
The Grimm are said to have appeared soon after the dawn of mankind. They targeted early Humans, seeking to destroy them and all of their creations. At first, it seemed as though they would succeed, as Humans did not have the strength to fight them.

However, Humans discovered the power of Dust and with it, the Grimm were driven back. During this time Humans enjoyed a time of peace and soon formed their own kingdoms, which grew to survive and prosper.

Although the old kingdoms eventually faded, the world of RWBY still enjoys a time of peace. But this time won't last forever. In the absence of light darkness returns, and the Grimm will soon make a resurgence. Additionally, the Grimm appear to be the predominant species in the world of Remnant, Humanity being limited to four pockets referred to as Kingdoms, which are guarded by Hunters.

Beacon Academy Initiation
As part of Beacon Academy's initiation, new students must team up in the Emerald Forest and fight their way to a temple, encountering several different types of Grimm in the process. Ruby and Weiss are attacked by a large pack of Beowolves, but are forced to flee when Weiss' attack causes a fire due to Ruby's accidental interruption.

Yang is also attacked, but by two Ursi that only manage to sever a strand of her hair, sending her into a blind rage as she defeats one of the Ursa, as the other is killed in one strike by Blake before Yang can finish it off. Likewise, Lie Ren is attacked by a King Taijitu, which he defeated through the use of his Aura.

While searching for the abandoned temple, Jaune Arc and Pyrrha Nikos find a cave that they confuse for the temple. While exploring, Jaune grabs on to the stinger of a Death Stalker, believing it to be one of the relics. Pyrrha then runs out of the cave and its resident, a Death Stalker, follows her out, breaking down some rocks covering the entrance to do so. Once outside, it flings Jaune, who is still hanging onto the supposed "relic", into the air and Pyrrha flees from the Death Stalker.

The Death Stalker chases Pyrrha to the temple while Ruby and Weiss are carried there by a Nevermore. Upon arrival, they meet up with Yang, Blake, Nora, and Ren, the latter two arriving at the temple via Ursa. The two Grimm chase the eight students to a larger part of the temple where Team JNPR are able to kill the Death Stalker by using a plan devised by Jaune: hacking off its stinger and then hammering it into the creature's head.

Meanwhile, Team RWBY were able to defeat the Nevermore by using Gambol Shroud as a giant slingshot to launch Ruby at the Nevermore. Using Dust capabilities of Myrtenaster and the recoil of Crescent Rose to propel her faster, Ruby successfully beheads it after dragging it up the face of a cliff. The defeat of the particularly fearsome Grimm using the plans of Jaune and Ruby prompted them to be made the leaders of their respective teams.

Episodes Covering These Events
 * The Emerald Forest
 * The Emerald Forest, Pt.2
 * Players and Pieces

Start of Classes
In the first class of Grimm Studies, Weiss, in an attempt to prove herself superior to Ruby, volunteers to take on a Boarbatusk in a class demonstration. In the battle, the Boarbatusk manages to disarm Weiss, but she eventually retrieves Myrtenaster to both block and kill it.

Later on, in the Forever Fall forest, it is revealed that there are larger Grimm creatures of the same name, although in Episode 6, a larger than normal sized Beowolf was briefly seen. In Episode 14, a much larger Ursa was seen attacking Cardin. Unlike the smaller ones, it had many more spikes, larger spikes, and armor plates on its back. However, it is defeated by Jaune with slight help from Pyrrha's Semblance.

Episodes Covering These Events
 * The Badge and The Burden, Pt.2
 * Forever Fall, Pt.2

Trivia

 * The name for the creatures of Grimm may be a reference to the Brothers Grimm or Grimm's Fairy Tales, a famous classic collection of German fairy tales compiled by the brothers.
 * When killed, some Grimm are known to emit smoke. This can clearly be seen in Episode 6, after Yang kills an Ursa, and in Episode 10, after Weiss kills the Boarbatusk. This could mean that when Grimm are killed, their bodies will eventually evaporate.
 * The shapes and swirls on Adam's mask are similar to the designs of the Grimm, possibly signifying some kind of connection between himself and the Grimm. It may also be an allusion to Adam representing the Beast of Beauty and the Beast.
 * The Grimm creatures' vocal sound effects were done by William Orendorff.
 * The Grimm, and their various subspecies, appear to be either common knowledge or a subject taught in combat schools. This is evidenced by the cadet Hunters being able to identify and name the Ursa and the Death Stalker by sight.
 * As of The Badge and The Burden six species of Grimm were introduced: the Beowolf, Ursa, King Taijitu, Death Stalker, Nevermore and the Boarbatusk.
 * In Episode 9, every known Grimm species (so far) was seen in sketches in Peter Port's lesson.
 * Whether or not the "birdie" that Ruby crashes into in Episode 5 should be classified as a Grimm is disputed.
 * There may be a market for selling Grimm pelts, mostly likely as trophies or decorations. Nora mentions a dream in which she and Ren make money by selling the pelts of Ursi. Whether or not this market exists outside of Nora's dream is unknown.
 * In Episode 14, a Grimm silhouette is seen in the credits, being an Ursa, and the first time a character from the show has not been seen.