The enormous and ancient cathedral that Salem called home possessed many deep catacombs, the product of it's original builders seeking to carve deep and hidden chambers, Grimm cleaving their way between the passages, and Salem herself either fortifying the endless labyrinth of halls or putting them to use. Being the Mistress of all Grimm though, Salem need not have ever feared the creatures of shadow, and so never bothered to remove them. This continued monstrous presence reinforced the impenetrability of the lowest levels of the cathedral, shielding the upper and surface levels from any creature, spy or unlucky soul that tried to reach the surface via Salem's sanctum.
This Grimm presence also made the tunnel's beyond dangerous to anyone not accompanied by Salem, a fact Watt had impressed upon Cinder not long after the young woman joined their cadre. Many a curious delver had vanished into the catacombs never to be heard or seen or hinted at again. If Salem was feeling particularly sentimental (an occurrence neither Watt nor Cinder had ever witnessed) or felt that the potential corpse could be of some use to her (the more frequent result) Salem would venture down and spend a few days checking rooms and halls for their body.
These points had been drilled into Cinder Fall's skull for hours on end, culminating to reinforce a singular rule for her to abide by regarding the catacombs: Do not go into the catacombs. It was stupid, foolish, and suicidal. She would die if she went down there.
So Salem felt a twinge of admiration when, in the midst of intensely hard work, she heard the familiar click of Cinder's ceramic heels behind her. "How many Grimm stood between you and this place," she asked.
Cinder walked up to a small wooden chest and plopped down on it. Her breath came in shudders and gasps, and blood was dripping from her palms. One of her swords was missing, and the other was deeply notched and in need of sharpening. "I..." Cinder shook her head. "Lost count."
"I can't say I'm not impressed," Salem admitted "I made a very good choice to bring you to me. It was... ambition that brought you to seek me out down here, and a drive to prove your worth, wasn't it?"
Cinder's panting, and her struggle to catch her breath, didn't abate nor allow her to answer. Salem nodded to the lack of answer. "I'll give you this, it's been over a century since someone last came down here and found me."
"Oh. Well... good..." Cinder said. She lowered her sword and closed her hands. The faint glow of aura pulsed in her palms before receding. The next breath she took was deep, filling, and more than a little relaxing. "I came down here because you were coming down here to take care of very important business, and I wanted to see."
"You wanted me to see you prove yourself in a crucible of shadow," Salem told her flatly. She walked up to Cinder and bent forward, just above eye level. "You came down here because you had the gall to believe that you were stronger than any who had come before, and the foolish desire to prove it to me. To make yourself more important to my plans, to leverage that into power." Her black and red eyes, soulless, drilled into Cinder's own. "Am I right?"
"D-Don't sell me short," Cinder retorted, off balance.
Salem cracked an intimidating smile, a demonstration of how well she could fake the emotions that souls conferred. "You're another piece. An important one, don't get me wrong, but a piece. Next time you're inclined to prove that, remember that I need every piece to play their part."
Salem stood up straight now. "You are to remain here with me. If you go out there in this condition, you'll die."
"I doubt that," Cinder said with as much false bravado as she could muster.
Salem resisted the impulse to roll her eyes: Human pride knew no bounds. She enjoyed using that. Pride before the fall, after all.
Cinder, now fully master of herself again, began to gaze in undisguised awe at the room she and Salem were in. It was a vast hemisphere of carved stone, with the lowest cycles given over to endless chalk formulae, diagrams, and notes. She saw images of a thousand Grimm, and as she traced each artistic rendering she saw the clear evolution from humble beginnings to the fearsome Beowolves, Ursas, Taijitus, and Goliaths that all Huntresses and Huntsmen knew so well.
Still to, she saw vast barrels and barrels, lining the walls, and while she was not inclined to look within any, she suspected that the contents would be a malignity shifting black sludge that, if too carelessly approached, would reach out to engulf her. Certainly there were enough of these barrels to form an army of Grimm, and being in such proximity to the raw stuff of shadow sent a shiver down Cinder's spine.
Curiously, the center of the room was given over to a vast and complex device made of Stone. It's primary feature was a pair of quarter-circles, opposite one another, in which were layered two full circles, all hanging below a large sphere, and above a well of undulating black sludge. As Salem approached it, mythic runes appeared in deep green coloration, and the machine seemed to hum to life.
The inference, which Cinder was now confident in calling a conclusion, was that she and Salem stood inside of what answered to the name of the Laboratory of Salem. A place from which endless horrors were wrought, to be duplicated and fully birthed in the shadowy pits high above and far below. Here was where Salem could mold that which would end the world.
Conscious of Cinder's awe and horror, Salem cast her a glance from across her shoulder. "Cinder, what do you know about the time before the Kingdoms?"
Thrown by the intrusion into her thoughts, Cinder took a step back and stuttered. "E-excuse me? Time before... before the kingdoms?"
"Yes. The time before the kingdoms. In that time, there were many lands, and many peoples, and many strange and horrible ideas." Salem's hand slowly traced an arc through the air, to encompass the central device. "In that time, enterprising Humans believed they could seize and remake the Creatures of Grimm, to make them as they were."
"That's impossible," Cinder said numbly, no conviction in her words. What she stood in, who she spoke to, what she saw... all impossible. Could there be any truth to her knowing what could and could not happen?
"For a time many agreed with you. Then, ages ago, the King of Lemure declared he would best the gods. He took the children of the God of Destruction, and imprisoned them for ages and ages. He tinkered with them, experimented with them, but as he grew old and weak, he found no answers. Then, in his waning days, he chanced upon a great discovery. He called upon the greatest warrior in the kingdom, and presented her with this device. He called it the Shadowforge."
"Into it she stepped, and when he had melted her down and reformed her, she emerged neither human, nor Faunus, nor Grimm. Yet his greatest creation knew nothing, but the all consuming shadow. She waged war on all creation, and only when she had slew the being she once called king and now called father, did she pause. She paused too long, and in a flash..."
Salem turned to the Shadowforge. "Her victory was thrown beyond her grasp."
"...So this is where you came from," Cinder murmured.
The Shadowforge twisted, turned. The two inner wheels rotated, spinning and curving within one another as the malign shadow below it rose up. The arching, pulsating form warped and twisted itself, before finally solidifying into a many tentacled horror.
Salem reaches forward and her shadow sibling wrapped her arm in it's tentacles. Without a hint of remorse or disappointment she watched the creature slough away into formless sludge once more. "One day... one day, the king will die. One day, there will be silence. And when there is silence, the Grimm will rule."
She turned to face Cinder, the Grimm's remains coating her forearm. She reached forward and pressed the shadow upon Cinder's brow, anointing her with darkness. "We will be Grimm. We will rule."