This is the story all y'all wanted! If you didn't read Betrayal, I would suggest doing that before reading this.
Contains spoilers (duh!)
~

My brain physically hurts.

Bill had just made me kill my family. I curl up into a ball and start sobbing. Something hits my head. I look up, tearfully, at Bill who was floating above me. "You're gonna have to forget about them, Pine Tree."

He puts a finger on my forehead and suddenly...wait, what was I talking about? Why am I crying? I wipe away my tears. Wait a minute. Who am I?

I swollow, looking up at the dream demon in front of me. "W-who am I?" I ask it.

If the demon had a mouth, I guess it would be smiling in a sympathetic way. "Don't worry," he says. "You're safe now. My name's Bill and you're my apprentice."

I swallow. "Apprentice?"

"Yes," the demon says, helping me up. "You're name is Pine Tree. I think you hit your head pretty hard if you don't remember that or me."

I blink. "What happened?"

Bill puts an arm around me and I have the strange urge to push him off, but I don't. "There was a terrible accident. I'm sorry to say but... you're parents didn't make it. I barely got there in time to save you."

"But what happened?" I ask urgently.

"You're car ran off the road and down a cliff," Bill says, his voice dripping with sympathy.

I swallow. "Oh." Was all I can manage.

"But don't worry," the demon says. "I'm a close family friend, and I'll take care of you. I'll help you master your powers."

"My...what?" I ask in confusion.

"You are a demon on the inside," Bill explains. "I'll help you unleash that power so you can use it to fight the evils of this world with me. And we'll be the rulers!"

"Rulers?" I ask. "What do you mean by that?"

Bill clear his throat. "Oh, nevermind. I'll show you to your room."

We get to the bedroom, which looks comfortable. "You should get some sleep," Bill says, floating in the doorframe. "You've had a long day."

I nod. "Thank you."

He shuts the door, but I swear I hear the lock click.

~

I scream in pain. "Stop!"

"It's the only way to harness your power!" Bill explains.

I stand in a glass container. My forehead where my Big Dipper birthmark is screams in white hot pain that all but blinds me. I feel myself scratching uselessly at the glass. Nothing I do seems to help. "Bill, please!" I beg. "I can't take it!"

Suddenly, everything goes still. Something is tightly wrapped around me. I open my eyes and look down at the blue chains wrapping me up. The glass has disappeared and Bill stands in front of me. "This is how you'll learn to control your powers."

"Then why does it hurt so much?"

"You're changing," Bill says. "You're going from human to demon. There'll be some changes, so I would suggest, man up and stop crying."

I hadn't noticed the tears slipping down my cheeks. I sigh. This was going to be a long day.

~

I ran as fast as I could. I couldn't let him catch me! Maybe if I run far enough, he won't be able to. It's been almost three months since I've been out of the mindscape, and I finally escaped.

Whenever I pass people in town, they give me this weird look, but it's more like they're trying to place my face. I quickly make my way into the forest, hoping that the trees will confuse him. Maybe he doesn't even notice I'm gone.

Suddenly, there's a flash of yellow light in front of me that forces me to throw my hands up to protect my eyes. "Nice try, kid," says a voice above me that makes me shiver. I timidly drop my hands, just as one comes out and strikes me across the face. I cry out in pain. "You're too dangerous!" Bill yells at me. "You're like a bomb! No one knows when you'll go off!"

I whimper. "Please," I beg, but Bill simply grabs me and teleports us into the mindscape. Then he drags me to the room where he trains me in. He grabbed me by the neck and holds me up in front of him. "Bad boys need to be punished."

Tears flow down my cheeks. "Please!" I beg again. "I promise never to disobey-"

"Ha!" Bill says, much too loud. "You're still human, and humans were made to break promises!" And with that, he throws me to the ground.

When I'm still stunned, something stabs me in the back. I scream in pain. It's too much. He's going to kill me!

The knife or sword was pulled out and I feel blood soaking the back of my shirt. I sobbed as this happens three or four more times then...I hear footsteps. Bill's leaving me here to die! I sob, hard.

The next thing I know, I open my eyes and I'm feeling a little better. The pain's gone, though I can still feel my blood soaked shirt sticking to my skin, cool air making it's way onto my back through the holes. I put a hand through one of those holes, surprised to find fading scars. I blink and sit up. Bill's gone and the sword he used on me sits there, it's blade now clean.

I wiped away my tears, but I jump when I look down to see, not water, but some sort of gold substance that feels like water on my fingers. I pick up the sword with shaking hands and peer into it. I drop it in surprise when I see my reflection. My eyes were glowing yellow with black slits and my hair is turning blond!

I scoot up to I can rest against the wall. I hug myself, sobbing. What's happening to me?

~

I tried running about once a month. Whatever pain Bill caused me, it was gone in minutes, no more than fifteen. I wasn't sure how much time went by, but I did that for about a year. My hair turned completely yellow and I had these spasms were I would blank out for a little bit, unsure of what I did in that time until the damage was done. I could control my powers and Bill went a little easier on me.

As soon as I mastered teleportation, it was easier for me to escape and allowed me to run at night, when Bill thought I was asleep. Only once did I have a moment of total panic and my mind blanked out when I was outside the mindscape. Bill had found my, shivering and dripping wet the next morning, a knife in my hand. Thankfully, it was clean.

This morning, I wake up with a weird feeling in my gut. Go to the hospital, a voice says in my head. I don't understand why I would go there, but the voice gets more urgent and loud throughout the morning. When Bill isn't  around, I teleport into town and run to the hospital. I didn't know where I'm going, but I run past everyone, them not even seeing me. I wonder briefly why they don't think it's weird for an almost thirteen year old boy to be running down the halls of a hospital in panic, clearly not having checked in. I'll learn later that demons, no matter what form they're in, are invisible to humans, unless the demons let the humans see.

I skid to a stop outside room 288. The nurse looks at me with a smile. She turns to the door and opens it. "You've got a visitor."

I walk into the room, and there's one name on my tongue, which I don't even know where it came from, but my brain seems to be thinking without me. "M-Mabel?"

~Time Skip!!~ (to figure out what the heck happens here, I suggest reading Betrayal, or y'all will be real confused!)

I swallow hard, fighting to stay conscious. My family from the other dimention had just disappeared and I was on my own. With a shaking hand, I ring the doorbell, the world swimming in front of me.

My mom opens the door. She stares at me, gaping. "H-honey!"

My dad comes up behind her. "Dipper?" He says in surprise.

It's about then when my knees buckle and everything went black.

~

I have vague memories of the next few days. I wake up in a fog, something sliding down my throat or something cool on my forehead. I can hear voices, but I can't make out who was talking or what they're saying.

When I finally fully wake up three days later, mom is dozing at me bedside. I don't want to wake her, so I stay quiet and still until she finally wakes up. A small smile spreads across her face. I can see the bags under her eyes. She smooths my hair back from my face. "Hey, Dipper," she says. "You feeling better?"

I blink a couple times. "You should hate me."

Mom stops stroking my hair. There's a look in her eyes that I know all too well: fear. My own mom, the woman that birthed me and fed me and washed me and was there when I needed her, is scared of me. It makes me want to break down. "We don't hate you," mom says, kissing my forehead. "We just need to get you a little help."

I gulp. "It wasn't me!" I say, shooting up. "I mean...it was...b-but there was this demon...he made me...I didn't want to...I miss her!" I started crying and I don't notice until I was done with my rant. I grab mom's shirt and sob into it. "Please don't get rid of me!"

I feel her nimble fingers in my hair again. They don't leave the whole time I cry.  This is all my fault! I made the deal. I held the knife. I...

"Shh," mom's voice is soft. "We want to help you Dipper, and we don't want to get rid of you. We love you, no matter what."

"B-but I'm a murderer!" I almost scream.  That makes me sob harder. "You should hate me! Why don't you?" I'm emotionally unstable. "You always loved Mabel more then me and I took her away!"

"Don't say that," mom says softly.

"But it's true!" I yell, jerking away from her. "I was an embarrassment anyway! Mabel was everything you wanted in a daughter, and I took her away!" I started to hiccup. I curl myself in a ball and sob harder, then softer. "I killed her." Then I get mad and yelled. "I killed her!"

Mom forces my head back and dumps something down my throat and everything goes black.

~

I wake up in a blinding flash of white. I blink a few times, trying to focus my eyes. I can't for almost three minutes. When they finally do, I look around. I'm in a hospital room, my arms strapped down at the wrists. I gulp, knowing exactly where I am.

There's a nurse in the corner of a room. She notices I'm awake. She smiles too brightly to be genuine. "Hello, Dipper," she says too cheerfully. "We're gonna help you get better."

"What, are you gonna "put me to sleep"?" I ask, making quotation marks with my fingers.

"We understand you're going through a difficult time," the nurse says. "We're here to help, not to get rid of you."

I can't help snorting. "What if I don't want to get better? What if I'd rather get a bullet through my brain?" I'm bluffing, but I'm not feeling too great at that moment. I hope I don't sound too crazy.

The nurse slowly makes her way over to me. "I understand that you're scared and confused."

"I'm not confused!" I yell. "I know exactly what's going on! You get possessed by one demon and kill your family and suddenly you land in the nut house!"

I see the young nurse swallow. She has a hand behind her back. "It's alright," she says softly. "No one's going to hurt you."

I snort. "Sure. And what if you can't help me? What if I'm too crazy? Then what?"

"You're not crazy," the nurse says slowly. "And we will get you back on track."

I gulp. I don't want to believe them. If I go sane again, I'll remember what I did and I'll have to live with it the rest of my life. I blink the tears out of my eyes. "It would be better if you just got rid of me when you have the chance."

The nurse was at my side by now. "None of us want to hurt you." And as she's saying that, she plunges a needle in my arm and everything, once again, goes black.

~

I woke up in the middle of the night, my mind foggy. My room was pitch black. "He's up," whispered someone behind me. 

Suddenly, a light was blinding me. I blinked a few times, but it was still really bright. "State your name!" Said someone to my right.

"D-Dipper Pines," I muttered sleepily.

"Did you or did you not kill your twin sister Mabel Pines and your Great Uncle Stanford Pines?"

I blinked. "Why do you need to know?"

"Because I'm the police," the man said.

I looked away from the light. "Yes, and no."

"What does that mean?" The officer grumbled.

"It was me, but it also wasn't," I tried to explain without sounding like a total nutcase.

"Tell me the story," the officer said.

"Get this light out of my face first," I said.

The flashlight was turned off and the normal, hospital ceiling lights came on. I glanced over at the officer. He had greying hair, a neatly trimmed beard and a mustache. He waved his hand, telling me to go on.

I took a deep breath and started my story. "It all started in Gravity Falls, Oregon..."