Some RWBY AUs

Shapeshifter AU

(Grimm is based on nekomata and drawing is made by me. Sorry for any bad cat anatomy.) There's a rare type of Grimm called the Shapeshifter. It has the ability to shape shift into recently deceased humans or Faunus and the power to shapeshift into humans or Faunus that it has killed and eaten.

If the Shapeshifter has successfully transformed into its human form after killing and eating a human or Faunus, that means it has also eaten the victim's soul, causing the person's spirit to be unable to rest in peace, until the Shapeshifter dies.

In its Grimm form, it can be the size of a simple house cat or a jaguar with two tails and Grimm features, such as bone-like spikes. It's much more deadly whenever it's in its human form than Grimm form. One of its significant traits they have is their intelligence and cunning; they can easily trick people into trusting them, plotting to kill and possibly eat them later. However, there is a way to stop this monster: you must point out their lack of aura.

Unfortunately, they're able to put up a "fake" aura to attempt to avoid suspicion. However, if you pay close, close attention, you would be able to sense their fake aura. When you show signs to the deceitful Shapeshifter that you know they're in disguise, it would immediately turn hostile to every human and Faunus, cat and Grimm body parts would show up on the Shapeshifter's human/Faunus body, and start attacking you. The friend/family member who you thought turned out be alive, now has black cat ears and two black tails, red markings over their skin, white spikes pointing out of there back, joints, and hand, really sharp claws and teeth, and a bone-like mask with crimson eyes.

The Shapeshifter would start attacking the people it "befriended". If the person that the Shapeshifter is trying to kill is still alive and fighting back or constantly running away, the Shapeshifter would start tormenting the person, trying to make them vulnerable to finish them off. The only way to defeat the Shapeshifter is to kill it, of course. After it died, its body would just lie there and evaporate into black smoke, like all Grimm. Whenever a Shapeshifter dies, often, all of the souls it has consumed in the past would  finally be released into the atmosphere, finally able to rest in peace. After the Shapeshifter vanishes into black smoke, several orbs of different colors would come out of the dark fog, flying up right to the sky. 

Minor fact: Most of the time, a Shapeshifter is caught when people sense them using no aura activity. They're also not able to use the deceased person's semblance, since Grimm have absolutely no aura. However, there's a drawback: it takes a longer time to kill a Shapeshifter than other Grimm. You must strike a specific spot on the Shapeshifter to cause massive damage to take it down quickly; other parts partially hurts it.

Here's another thing: the Shapeshifter might be clever enough to escape. If injured or it knows it's trapped or cornered, they would attempt to run away. If they do, they would transform into their Grimm form and run around the obstacles and escape. When it's escaping, it's vulnerable to most attacks. If they tried to escape but failed, they would continue fighting their threat. 

Random behavior fact: If a Shapeshifter is in its human form, it must go outside at night and cause some destruction once per week. If it doesn't, it would start behaving out of character, specifically more Grimm-like. If it still endure its instincts, it will go insane, all of its Grimm features would show up, and will kill something or someone. Often, the "person" would go for a "walk" at night and would kill an animal or two to relieve their bestial instincts. After all, all Grimm will attack humanity when possible. It's just that the Shapeshifter has patience and experience to control its brutal habits.

Random combat tactic: It's highly likely that the Shapeshifter that people would encounter have killed other humans and Faunus in the past. When fighting a threat, the Shapeshifter would often use its past victims' form and appearance to attack and distract its enemy, making this Grimm a huge challenge to defeat. Also, depending on its current form, depends on its current strengths and weaknesses. For example, a Shapeshifter using the form of a rabbit Faunus would be able to jump pretty high and be able to dodge attacks with not much trouble, but get severe injuries easily when it gets hit by an attack since it has a fragile body currently. 

A minor story on this AU:

"Y-Yang...? Wh-What's going on...?" Ruby stuttered. The two girls are in a middle of the forest in the middle of the night, along with Weiss and Blake. The bright, shattered moon in the sky makes Yang's crimson red eyes sparkle in the midst of darkness. Wait, crimson? 

"Well, well. Guess you heard that scream, huh?" Yang replied in a casual tone. She walks forward, making herself more visible. Ruby, Weiss, and Blake gasp in horror and shock when they see the corpse of a young man that Yang is carrying over her shoulder. The man's clothes are stained with his blood and his body had a huge, gaping hole through his chest with a bit of his white ribs showing out. "He seemed to be lost here and he happened to have the misfortune to meet me in the dead of night," Yang said as she threw the body to her left, disgusting and terrifying the hell out of the other three huntresses.

"Yang, what the hell has gotten into you-" Weiss is cut off when the blonde teenager suddenly throw the man's heart towards her, causing Weiss to shriek loudly. She barely avoids the organ and it lands onto a patch of grass, still beating.

"Looks like my midnight snack is postponed, thanks to you nosy girls," Yang said as she takes one step forward, causing the three teens to snap out of their petrification and get into their battle stance, preparing their weapons. "Let's just get this out of the way, alright?" The other three huntresses stand in confusion for a bit, but they start to witness something bizarre happening to Yang's body. Red markings started to rapidly form over her skin, two rigid spikes sprouts out of the back of her hands, her nails transforms into large, sharp claws, two black cat tails grows out of her tailbone, two cat ears grow out of the top of her head, and a bone-like mask with scarlet imprints covers her eyes.

Ruby, Weiss, and Blake stands there in shock, not easily taking in on what just occurred. "This is certainly an eye-widener, huh? It's not surprising for you three to look like that like you just saw a ghost." Yang's voice is suddenly distorted, but still understandable.

"Who a-are you?! You're not Yang!" Ruby shouts out loud to the transformed brawler.

"No shit, Sherlock that I'm not your half-sister or whatever," This "Yang" said in a rather cheerful tone. "I killed this Yang that you humans know and love and took her form. She was a such an idiot that she was unable to sense me when I was near her."

The three huntresses are once again stunned to what they just heard, looking like as if they were struck by lightning. KILLED?! TOOK HER FORM?!

"Still speechless, eh?" The impostor said in an amused tone. "Remember during one of the classes the teacher stated that there's a rare Grimm that can shape shift into people and Faunus it killed and is called the Shapeshifter?" She spread her arms apart, her right hand covered in the man's blood. "You're in the presence of one right now, which also killed and ate Yang Xiao Long, the unfortunate bimbo who died at my fangs and claws~." The Shapeshifter sneers wickedly, enjoying the terrified faces of the fools who she befriended. "They chose a pretty cool name for my species and let me give your friend a little compliment. Her meat tasted rather burnt, but still tender and enjoyable~." She said, adding more salt to the wounds. 

Tears start to form in the three teenager's eyes. Yang, who they thought was alive and herself, turns out to be a monster who killed her, ate her, and took her form.

"Back to the important matter, you guys interrupted my midnight snack. However, there's a solution for this accident." The cat-like Grimm looked over the three frightened and astonished teens, licking her lips. "You three will become my midnight dinner. Then I'll forgive you."

Horrified and disgusted expressions appears on the girls' faces. They won't let this monster kill and eat them just like that. Just the mere thought of it  sounds awfully gruesome and vile.

"We won't let you do that easily," Blake said while attempting to prevent her voice from shaking. She swiftly changes Gambol Shroud into its pistol form and starts to shoot at the deceitful Grimm, who easily dodged the bullets in a matter of seconds. The Shapeshifter appears behind the cat Faunus, covering  her mouth from behind and using her other hand to grab her stomach, the sudden pain causing Blake to flinch. 

"It's a catastrophe that your girlfriend died, isn't it~?" The Grimm said in a sympathetic tone right next to Blake's ear before digging her claws into her stomach.