Mabel is a fair bit older here. (Around ten years)

I do not own Gravity Falls.

"B-Bill?" Mabel called through the suffocating darkness. "Is that you?"

She took a step back only to freeze when her body hit into the front of another. Bony fingers gripped her shoulder tightly, and she felt a hot breath next to her ear.

"Why of course it's me," she heard Bill's voice coo into her ear. "Were you expecting anyone else, little Shooting Star?"

"Where are we, Bill? Wh-What's going on?" she asked, clearly frightened, as she attempted to get out of the demon's harsh grip.

"What's the matter, little star? Don't you like spending time with me?" Bill's voice became harsher as he spoke.

"You're acting weird, Bill!" she grunted out as she continued to struggle. "And you didn't answer my questions!"

"Why, we're just in the forest!' his voice was soft again, though somehow that made her feel even more fear than before. "And I just made sure you can stay with me forever!"

"What do you mean-" her voice died in her throat as the forest was suddenly filled with light, revealing the horrors it had shaded from view.

The bloody and battered corpses of everyone she cared about hung from the trees, rocking slightly in the wind. As her eyes widened, Bill suddenly pushed her forward, causing her to fall on the rough ground. As she looked up, she discovered she as directly underneath Dipper's lifeless body, his eyes glazed over.

"See, they would have stopped you from being with me," Bill hissed in her ear, bent down next to her. "And that just wouldn't do. You're mine, Shooting Star."

"B-but-"

"Mine..."

Her eyesight blurred, and at first she thought it was due to the tears pooling in her eyes. However, when the colors around her seemed to mix and blend like paint, she became confused, though Bill's voice still echoed through the forest...

Mabel shot up in her bed, her brown curls flying around her with the swift movement, a scream ripping through her throat. As she breathed heavily, trying to calm herself down, Bill sat up next to her, his uncovered eye full of concern.

"Are you alright?"

Mabel froze as he spoke, her eyes widening in fear.

"Mabel? Shooting Star?" he had his hands on her shoulders now, though it was a much more gentle grip than in her dream. "Did you have a nightmare?"

When she nodded, he scowled.

"Augh, I should have made sure you didn't get any. I am so sorry, I as being lazy tonight after being so exhausted from the bachelor's party your brother planned," he rambled, obviously frustrated at himself. He stopped when he noticed her nearly crying.

"What exactly happened in it?" he asked hesitantly.

"You-you killed them, said I w-was yours alone," she whimpered, tears starting to roll down her face.

"I-I would never do that," Bill eventually said, stunned by what had been in her dream.

"It seemed so real.."

"Mabel Pines," Bill suddenly lifted her chin, looking directly into her eyes. "I, Bill Cipher, promise you that I will never do anything that would hurt you or anything that you care about."

"... That's a pretty big promise," she said hoarsely.

"And it's one I intend to keep," he responded firmly. "Anyway, seeing what's coming up on Saturday, it seems big promises is the theme of the week!"

Mabel leaned into Bill, and allowed him to rap his arms around her.

"Yeah..."

She closed her eyes, banishing the memory of her nightmare from her mind as her fiancée gently kissed her forehead.