The Last Ace

"...Lucifer? Are you okay?" asks Ruby.
Lucifer didm't say anything. He slowly turned from the direction Yang was knocked from his punch to sweep over team RWBY and JNPR, and once more Blake, and the rest, felt the chill down their spine, the feeling of total helplessness, the faint knowledge that they are about to die.

Kicking up another dust clout, White-Haired-Lucifer charged at them with surprising speed, one arm drawn back for a punch.
"Spread out!" yelled Ruby, and the group of 7 became individuals as they went in every direction, making more difficult targets for White-Hair. He stood at the center of the swirl of running people for a second, before he randomly chose a target and charged.
Said target was Jaune, who was terrified at the sight of his approach.
"He's after me!? Why me!?" he was wailing, running as fast as his legs would carry him.
"It's called 'weeding out the weak', Jaune" said Pyrrha, and fired a shot from her weapon, catching him on the side of his face, knocking him off his course, Jaune grateful enough for that he didn't even say anything about her remark. However, White managed to form a mask of Exo for protection at the last minute and prevent any serious damage.
However, anyone could tell he was seriously pissed at the unexpected attack.
He stood erect, and glowered at Pyrrha, sending a mental promise he would be the death of her that chilled her to the bone. She started to run, but he was upon her in an instant, forming Morph Claws and raising them high. In half a minute, she would have be torn into strips, were it not for Ruby, who fired another shot from her weapon blasting him away from Pyrrha.
As he stood up once more, Ruby decided that running won't do it.
She yelled "Keep moving! Don't slow down! And fire on him from every direction, but try not to hit one another!" The effect was immediate, bullets and projectiles came at him, creating huge dust clouds that dissipate as fast as they are formed from all the mini-explosions of Dust ammo.
White turned around aimlessly in the pandemonium of projectiles for some time, before the anger became to much and he roared mightily at the sky, a roar of anger, bloodlust, and hatred, and he chose another target to attack.
He never got the chance, Ozpin appeared right behind him, saying "That would be enough of you, Lucifer." and whacked him on the back of his neck with his cane. Everything became black.

- Where am I?
Lucifer is standing in a place he has never seen before. The sky is black, and the earth is white. And Lucifer is glowing with an odd white light that seemed to pulsate along with his heart.
- What is this place?
- This is your mind, fool. Don't you know your way around your own damn mind?
- Who's that?
It was Lucifer, but it wasn't Lucifer. He is surrounded by darkness, and seemed to drown out all light around him.
- Who are you?
- Who am I?
He smiled, a mad, dangerous smile of someone looking down on a helpless person and wondering how was he going to kill him.
- I am you. I am the rightful owner of this body. I am the more worthy between us of control. I am the one who is better than you.
- The Grimm?
- Yes, that's what I am.
- You are supposed to be behind bars.
- Yes, I was, but when the blonde maiden attacked, the bars were removed.
- Then I'll get you back into the corner and re-erect the bars, Grimm. 
White Lucifer formed a Morph Blade and charged...
...And passed right through him.
Black Lucifer laughed and hit White Lucifer on his head, causing him to fall.
- You're trash, Lucy.
- My name's LUCIFER, Grimm!
White Lucifer swung, and the blade went right through him as if he weren't there. Black Lucifer laughed again. He punched White Lucifer, causing him to tumble across the land. White Lucifer didn't get up, he just lay on the ground writhing in pain. The blow rivaled that of a runaway train ... one with explosives on the hood.
Black Lucifer went over and kicked him.
- Trash.
Kick.
- Worthless.
Kick.
- Weakling.
Kick.
- Cowardly.
Kick.
- You would have been much stronger...
Kick.
- ...Avoided much pain...
Kick.
- ...If you would just...
Kick.
- LET.
Kick.
- GO.
- Go to hell, Grimm!
- That's not my name. 
Kick.
- You know my name. 
Kick.
- My name is...

"NEIMO!!!"
Ruby, Weiss, Blake, Yang, Jaunce, Nora and Pyrrha all flinched at that. Only Lie, Ozpin and Glynda remained indifferent.
"What? What did you say? Who's Neimo?"
Lucifer wasn't listening. He was sitting up and staring straight ahead, as if seeing the door to Hell in front of him and swearing he heard a creak of the door opening, and making sure the door remains closed.
"LUCIFER!" yelled Ruby, knocking him out of whatever thoughts he was having "What are you saying? Who's Neimo?"
"Neimo..." muttered Lucifer "Neimo ... is me."
"What?" says Yang.
"Neimo is the Grimm. He-" he gulped "He is the one within me."
"What?" echoes Weiss.
"Remember what I said about the lab coats simply chasing Him to a corner and erecting bars around him? He said that when Yang attacked, the bars were removed and he was free."
"I would seem" says Ozpin, simply looking on until now "That Lucifer is at least part Human. Surely enough Human to be able to have Aura. I would guess that when Yang attacked with her Aura Semblance, she involuntarily unlocked Lucifer's Aura, and subsequently the mental bars holding this 'Neimo' was removed. I guess its safe to say that whenever Lucifer's Aura is activated, Neimo will emerge."
"So all I need to do I avoid using Aura?"
"Basically, yes."
"What do you man by 'basically'?"
"I mean it's better insurance to, uh, train, the use of your Aura."
"What?"
"Avoiding the problem won't save you forever. It's best for you to face it. If you keep Neimo locked up, He is bound to escape again sometime, and when he does, he'll kill everyone close to you."
"So what do I do?"
"That" says Ozpin "Is a question for you to answer."

The next day, despite the previous day's events, Lucifer was present. Team RWBY and JNPR met up with him during lunch.
"Sooo..." begins Ruby "Any idea what should you do?"
"A vague one, to be sure, but yeah. I'll activate my Aura once in a while in you guys' presence. I want you blast me when I attack, and stop when I do. That may send the message that you mean no harm, but won't hesitate to kill me if I go too far."
"Okay..." says Yang "...Now onto your real plan."
Lucifer stared at her.
"I'm kidding, but really, are you sure that will work?"
"Honestly? No. But Ozpin's right, just locking him up isn't going to work forever. This isn't a card I'm playing because I don't know what else to do, I'm playing this card because this is the ONLY card I have. I don't want to hurt anyone."
The rest of the day passes without incident, and Lucifer says he wants to go to the library after school. "I didn't have all that many reasons to read up on Aura and Semblance back at Signal because I thought I won't be having it, but since I do, might as well look up on everything there is to know about it."
Lucifer returned to his room - actually more like an unused broom closet - later in the day, grinning from ear to ear.
"What's so funny?" asks Yang.
"Check this out." he says, and hands them his rifle.
Holding up the rifle, Yang and the rest try to figure out what was different with the rifle. Then Ruby noticed a glint. "Oh, no. You didn't"
"I did. Engraved it myself."
"What?" asks Weiss, who didn't know what Ruby saw.
Ruby placed a finger on the rifle, just beneath an engraved metal plate. On the plate were the words: 'Justabasic Rifle.'
"Heavens!" chuckled Yang.
"Are you crazy or immature or what?" says Weiss.
"This is stupid." says Blake "You're stupid."
Lucifer laughed. Then he said "Tensions were kinda high after yesterday. It was bad. I...I couldn't feel comfortable, you know? Had to cut the tension somehow. Caught sight of the Forge on the way back from the library. Lightbulb above my head just went 'BLING'!"
He laughed again, and the others joined him.