(I drew the picture, and there's more in it but I can't show you guys yet;))
Reader/Bill POV

Mabel stared at Bill, eyes wide and mouth agape, "Dipper? What did you do?"

"Bill saved you, Mabel, you're alive because of him." Dipper said, smiling.

Bill didn't feel right, something was off.

Mabel's heart monitor sped up, and her face turned pale.

Dipper stood up and rubbed his eyes furiously.

"Dipper what's wrong?" Bill asked, suddenly concerned.

Dipper screamed in agony and he held his face in his hands, still helplessly scratching at his eyes.

Mabel fell back on her bed, black slime once agin returning to the corners of her mouth.

Bill looked back at Dipper, his eyes were gushing the black slime. Bill stumbled backwards.

Mabel and Dipper screamed at the same time. And then it stopped as Dipper fell to the ground, eyes wide open as tears formed in them.

Mabel stayed silent.

Bill saw a button on Mabel's bed that said the word 'NURSE/EMERGENCY' he frantically pressed it.

A group of three nurses barged into the room, "What happened?!"

Bill looked at them shocked, fumbling to say words.

The nurses pushed him out of the way, checking the vitals of Dipper, unconscious on the floor. He saw the black slime come off of Mabel's bed and begin to form a being.

"You must be the famous Bill Cipher. How pleasant to meet your company. And don't worry about the two unconscious humans behind us, I just gave them a little bit of nightmare fuel, maybe a vision." They said.

Bill looked at the being, "what are you?"

"I am Cix, the demon of unholy nightmares and visions." They smirked. "Kinda like you, Cipher, just not as powerful, and you control all things chaotic! That must be quite the job, running the nightmare realm!"

Bill clenched his fists, "You must be new..."

Cix laughed, "Yes, in fact I am! How kind of you to notice! But I wasn't made serve you."

"Then who?" Bill asked the other demon.

"Simply put, I serve myself. I don't follow a commander's rules. I was made to spread terror through anyone I could find." Cix replied giggling. "And my strength is nothing compared to yours or any other demon barely surviving without you in the nightmare realm."

Bill raised his eyebrow, "Barely surviving?"

Cix just laughed and flowed back into Mabel's body, entering her mouth.

Bill opened his eye to a bright light being shined in it.

"Sir are you ok?!" A nurse asked him.

Bill realized he was lying on the floor, "yeah. I'm fine." He swatted the nurse away, and looked over to see Dipper and Mabel in perfect condition, Dipper on a chair and Mabel asleep on her bed. "How long was I out?"

"Only 5 minutes sir, now sit down please"

READER/Dipper POV

Dipper tried explaining to Mabel how Bill saved her life, but his eyes began to burn furiously. He stood up abruptly and rubbed his eyes. He heard Bill ask him if anything was wrong. Dipper opened his mouth to reply, but his eyes felt like they were being stabbed, he screamed and scratched at his eyes, hoping to get something out of them.
He heard Mabel fall back on her bead and he screamed again, this time hearing Mabel's cries join in as well. Dipper felt his body give way as he fell backwards he could only see black.

Dipper's eyes opened to the forest, he frantically searched around for Mabel and Bill, wondering how he got there.

"How nice of you to join me." A familiar voice called.

"Who are you?" Dipper called.

"That's none of your concern human. What is your concern is that, as of now, your sister's body is mine, and so will you be eventually." The voice said.

"What did you do to her!!?" Dipper cried out. The demon appeared suddenly behind him, he sharply turned to face it.

"You remember when I "killed" her? I was really just transferring my essence to her being. And when I pushed my hands onto your eyes I gave you the ability to see horrible visions at my command!!" The slime demon laughed. "And thats what your having now."

They snapped and Dipper watched in horror as his surroundings changed to show Mabel dying over and over again, each time dying a little differently. Sometimes being decapitated and other times being caught on fire. Dipper was forced to watch his sister die thousands of times over.

His eyes burned once more as he awoke from the nightmares, only to see nurses surrounding Mabel, Bill and himself. Dipper stared blankly at Mabel, "I'm so sorry, I should have just let you die."

He saw Bill gently push his way through the nurses and doctors and go towards him. Bill gripped his shoulders, "What did they show you?!"

Dipper looked at Bill, "Mabel... she.. died over and over again." He felt tears go down his face and he looked down to see black slime covering his shirt and hands. "What happened Bill?"

"Nothing good Pine Tree." Bill replied.

READER/Mabel POV

Mabel felt her abdomen begin to sting, and she felt her consciousness slowly slip away as she collapsed on the bed, screaming, but it stopped as she woke up again.

The demon walked in front of her bed, leaving black slime in their path.

"Hello Mabel." They said smiling.

"Who the hell are you?" Mabel asked.

"Right now? I am slowly becoming you, your soul and mind are deteriorating, and your body will soon become my new vessel. My name is Cix, nice to meet you, Mabel Pines." The demon held out its hand to shake.

BACK TO READER/Bill POV

Bill explained to Dipper who the demon was as what it was planning to do to his sister and him. He gripped onto Dipper's shaking shoulders, keeping him conscious.

Dipper cried, quietly saying "I should have just let her die I should have just let her die I should have just let her die"

Bill reared his hand back and slapped Dipper's cheek. "Shut up dammit!! If you had just let her die she'd be in worse condition than she is now!! If she was dead that would just let Cix have her body, no problem! If she stays alive, she can fight and we can look for ways to help her!!"

Dipper swallowed the lump in his throat "Ok, what do I have to do?"

A/N

This chapter was was published later than I
Intended, so sorry about that. I have plans for the Cix and everything and I'm so excited. And yes, BILLDIP WILL HAPPEN DONT WORRY MY PRECIOUS CINNAMON ROLLS!!! And here is the rest of the picture from the beginning of the chapter:

Sorry for the crappy quality. It's on my wall that's at an angle.

~ DURPCAT OUT!!