Disclaimer:I don't RWBY nor do I own Undertale. They belong to their respective owners. I realize that I left the first chapter in an awkward place, but I'm too lazy to edit it so I'm gonna leave it as is.
….
It is late at night at Mountain Glenn, and there is grimm prowling throughout the quiet night. The only humans you can find on the mountain are four teenage girls and one middle-aged man, who are currently asleep. The only fully awake person is a blonde seventeen year old girl, named yang.
Everyone was asleep until a black corgi, who was sleeping next to a young fifteen year old red-head girl, named Ruby, started to stir from his sleep. This action subsequently woke up his owner. Ruby slightly startled from being pulled out of her dreams, looked around only to see her dog run off.
"Zwei...Zwei...Zwei, where are you," Ruby whispered, not wanting to wake up her teammates. She left her group to find the corgi, and found him urinating outside.
"Zwei. This is a wasteland. You literally could have done that anywhere," Ruby scolded the dog. As she was picking up her dog, she heard male voices close by. Ruby quickly went to hide behind a building and scanned the area to see who was talking. There were two members of the white fang talking to each other. They were patrolling, and Ruby decided to tail them with Zwei casually following her.
Ruby followed them, until they went inside a building. Ruby took cover behind another building. She picked up her corgi, and had him look to see if the two white fang members went inside a building.
"Did they go in yet? One bark means yes, " Ruby asked her dog. The dog barked once
"Oh! This is it! This is it," She told her dog excitedly. Ruby pulled out her scroll to call her teammates for backup, but there was no signal.
"Oh man," Ruby said disappointedly, but she had to see what the white fang were up to and find their secret hiding place. As she was walking towards the building, Ruby heard cracking sounds and saw that the floor was braking. Ruby realized she was standing in a sinkhole too late. The floor collapsed. Ruby tried grabbing an edge of the floor, but her hands barely grazed it.
She along with Zwei fell down the deep hole. Ruby grabbed Zwei by the scruff of his neck and pulled him close to her chest. The corgi was whimpering in fear. Ruby closed her eyes shut, she had no idea what to do.
….
When Yang's shift as look out ended, she stretched out her limbs and walked to the others. She was going to wake up Weiss, but noticed Ruby was not there.
"Hey where's Ruby," Yang asked. Everyone woke up from their slumber.
"What," Professor Oobleck asked, popping out of nowhere.
"I don't know," Blake responded, rubbing her eyes.
"Hey, look Zwei's not here as well," Weiss said, looking around for the corgi and her red themed teammate.
"This is bad," Oobleck said, while rubbing his chin, "Grab your weapons. We must search for Ruby and Zwei at once. For all we know, she could be in trouble."
Everyone went out into the night to search for Ruby and Zwei. They were all worried and hoping that nothing has happened to the youngest member of team RWBY. Team RWBY and Oobleck split up in pairs in order to cover more ground. Weiss went with Oobleck to search for her partner, while Yang went searching for her sister with Blake.
After two hours of searching for Ruby, Yang found Ruby's High Caliber Sniper-Scythe, dubbed Crescent Rose, near a hole. Ruby was nowhere in sight, and Yang was going into her overprotective sister mode. Blake was contacting Oobleck and Weiss about their discovery.
"DAMMIT! She must have fallen down the hole," Yang exclaimed frustratedly. She bunched a building nearby, leaving a huge hole in the building. "I must be the worst sister ever…"
"Don't say that Yang," Blake told her, placing a comforting hand on her shoulder. Yang looked her partner in the eye. The intense stare the two girls shared was broken when Weiss and Oobleck arrived at the scene.
Oobleck began explaining to the girls the history of how the people of Mountain Glenn moved underground to survive from the numerous grimm attacks. The professor also told the girls how the people of Mountain Glenn all died in the underground village. He, along team RWBY, realized that the underground crime network they were searching for was literally working underground.
"If Ruby is down there...we must find her," Professor Oobleck said, transforming his coffee thermos into his weapon.