This probably should have been in the last chapter but oh well, here it is:
(also this chapter hasn't been checked over, so please tell me if there are any mistakes and I'll correct them.)

Straightening up, the twins started to follow Bill deeper into the maze of trees, tripping every so often from the lack of light and hindered vision; Dipper glanced at his sister occasionally to check that she was ok and she seemed to be holding up well, looking as though she hadn't been close to coughing her lungs out not long before. She was noticeably quieter than usual.
"So, uh…" Dipper coughed to clear his throat, "what are you going to show us?" he coughed again, more heavily now and Mabel sneezed next to him. Bill ignored him and continued to float along the path. "Bill?" he asked again, a trace of annoyance slipping into his tone and nearly bumped into the dream demon when he stopped suddenly; Mabel bumped into his back, releasing a soft grunt.
'Alright, now be quiet, kid.' Bill's booming voice filled Dipper's head and he jumped at the sudden intrusion; the words left a tingling feeling numbing his brain and he felt the urge to scratch at it. The immortal visibly dimmed his yellow glow to a softer shade that wouldn't draw so much attention to them. They crouched behind a tall bush and Bill floated down next to him.
"Why, what are-" Dipper whispered before being cut off by the voice in his head;
'Just think what you want to say, Pine Tree. I can read your mind.'
Dipper decided to worry about that invasion of privacy later; 'what are we looking for?'
'The acheri; it's around here somewhere.'
A violent twist pulled at Dipper's stomach and his hands shot to the area; he grunted in pain as a sharp stabbing sensation in his abdomen made him curl up into a ball, whimpering quietly. Bile rose in his throat and he lifted his head slightly to release the contents onto the grass in front of him; a pair of warm, comforting hands were on his back and he suspected that Mabel was trying to comfort him. The warmth was wrenched away as he heard his own sister vomiting next to him; he'd have comforted her if it weren't for the lingering pain that paralysed him.
'Here, kid,' Bill thought to him. Dipper moved his head slightly to witness the demon's hand light in a blue flame for only a split second, and in him palm was now two red strips of ribbon; Dipper furrowed his sweat-drenched brows in confusion. 'Tie this around your neck, it'll protect you.' He realised that the thirteen-year-old boy was still unable to move and opted to resort to controlling him himself. He tapped into Dipper's mind carefully so as not to cause too much more discomfort for the teen; once there was a small opening in his mental barrier, Bill squeezed through the tight gap and set to work.

The aching in Dipper's limbs dulled a small amount and he stayed still for a moment before his hands lifted up… But Dipper hadn't controlled them; his fingers twitched and wiggled for a moment and a grin lit up his face, but not of his own accord. His hands lifted one of the red ribbons and the fingers danced over the material briefly, seemingly relishing the sensation of the soft fabric against the pads of his digits. His hands brought up the strip of fabric and wrapped it around his neck; tight enough not to come loose or fall off but loose enough not to choke him or hinder his breathing – a knot was tied at the back of his neck and his hands fell away. Picking up the second strip of red ribbon, the appendage lifted it and his head moved to look at his sister who was curled in a tight ball, eyes locked shut tightly and hands shaking, gripping opposite arms over her stomach. Moving the red strip, his hands wound it quickly around the brunette's. His hands tugged the ribbon sharply, causing the girl on the floor to flinch but pay no other mind. Just as the knot was tied, Dipper felt himself regain control of his own body and he wiggled his fingers cautiously. He didn't fail to notice that the horrendous pain had ceased – he didn't feel the urge to throw up his viscera, wasn't coughing up the mucus in his airways, wasn't curling into himself from the murderous stabbing sensation in his abdomen; he felt a lot better.

'Heh, sorry Pine Tree; had to use the meat-sack you call a body to do that else you'd be in a lot of pain. And as fun as that sounds, I can't go breaking my end of the deal.' Bill's voice filled Dipper's head again. Next to him, Mabel was slowly leaning up again, her own pain appearing to have disappeared as well.
'What did you do?' Dipper thought.
'The ribbon protects you, so don't take it off unless you want a slow, diseased, painful death.'
The boy's mouth went dry at the reply and he asked; 'protect us from what?'
The dream demon pointed a small finger above the bush that they were still crouched behind; Dipper looked over the top and was surprised at the revelation: a small girl.

The girl looked to be only ten years old, her ligaments were deathly thin and pale and looked to be bones covered in a thin layer of paper. Feet bare as she stepped slowly through the forest, she donned a thigh-length piece of clothing that was probably made from animal hide. Greasy, black locks of hair fell down to her shoulders and framed the hollow features of her face; which was sunken in to look like a skull. A headband wound around her head and held a grey feather that brushed against stray tree branches and leaves while she walked; the leaves darkened, shrivelled and fell to the floor at the contact from the feather. Dipper noticed, then, that the grass below her feet yellowed and became limp as she crushed each blade beneath her. Next to him, Mabel gasped quietly when she shifted to look at the girl, too. "Who is she?" Mabel asked and the twins looked to Bill expectantly.
"It's an acheri." Bill said unhelpfully.
"And what's that?" Dipper asked. In response, the dream demon lifted a spindly arm and tapped the side of the boy's head twice before looking at him; the male teen jolted as his vision blurred. He looked into Bill's single eye as a fountain of information blurred into Dipper's brain; he reached up to rub at his head when a dull ache settled in it. He looked up at the girl in the clearing, his vision clearing up quickly, she had now turned to look in their direction – her eyes were sunken into her face and were a deep black and Dipper gasped again when he found that he knew everything about this girl – this spirit.
He frantically pulled out his journal from his vest and opened it to a clear double-page spread. Scrambling for a pencil in his shorts pocket; he found one and began scrawling messily in the book: he sketched a small drawing of the girl on the left page, writing 'acheri' in large letters above. On the right page, he scribbled down the newly given information from his brain.
"Dipper," Mabel whispered, "what are you writing?" he gave no reply, focused intently on writing before the information left his brain and was forgotten. She looked over his shoulder, reading what he wrote aloud with a frown; "spirit of a young girl who comes from the woods once a year to bring sickness to people, mostly children. Also called 'the hill fairy'. The only defence from the acheri is to tie a red ribbon around your neck; this protects you from the disease she brings." Mabel reached up and traced her fingers along the ribbon around her neck. "Can bring death to the elderly or already sick." Mabel frowned at the information. "How do you know all this?" she asked less quietly; he gave no reply but did glance at her, a small smile adorned his face and his eyes were alight with a new discovery.

Rustling could be heard not far from in front of them and Dipper finally stopped writing to glance over the bush; the acheri had started walking slowly toward them. A slight panic began to settle in his mind; if that thing was chasing them, how was he supposed to write about it? What if he forgot it all before they could make it back to safety? He leaned down again, writing faster than before.

"Time to go, Pine Tree," Bill said.
"Wait, just let me finish this," Dipper argued. Mabel glanced over the bush, too and saw the spirit getting closer: it definitely knew that they were there, now.
"Dipper, we need to go," Mabel urged him.
"Wait, wait," Dipper defied them both.
"Pine Tree," the dream demon warned again, "I can't let you get hurt; go now or I'll have to get you out of here myself."
"I'm nearly done," Dipper stated before his senses numbed again and the pencil fell limply from his hand. He urged his body to pick it up but it didn't listen; on the contrary it closed the journal, tucked it into his vest, then stood quickly and glanced at the acheri which was now walking toward them at a steady pace. The bushes and trees shied away and seemed to shrivel from her presence and a scowl was etched in her face as she had probably noticed that the twins weren't affected by her presence themselves. Bill turned Dipper's body sharply and started walking quickly back into the line of trees they'd come from.
"Dipper," Mabel called from behind him, clearly offended at being left behind.
"Be quick, Shooting Star," Bill said through Dipper's voice, "I'm not responsible for what happens to you~" he sang joyfully, not bothering to look back at her.
Dipper wanted to scream at Bill for being so ignorant but was helpless- the demon had full control of his body.
"Might not want to stick around there too long, kid, you might have that ribbon but it won't protect you from her physical attacks!" Bill called back happily before starting into a jog; Mabel soon caught up and was jogging alongside him. Glancing back, Bill noticed that the acheri was now sprinting after them at full speed, quickly gaining on them. He laughed and ran faster, Mabel followed suit.
"Dipper! What's wrong with you?!" Mabel panted and glared at him.
Bill laughed again at her before he felt Dipper's body get shoved and knocked over from behind; he spun over onto his back and looked up at the acheri that had tackled him to the floor. She climbed over him and lifted a hand to claw at Dipper's face. Bill smirked and caught the hand that flew down at his vessel; he lifted dipper's other hand to go around the girl's throat, which she hissed at ferociously. The putrid stench of decay invaded his nostrils when she opened her mouth and Bill's smirk widened as he put more pressure on her neck, lifting her up off of him only slightly; her arms flailed as she tried to claw at him, he lifted his other hand to wrap around her neck and lifted her up further. She scratched at his arm and small droplets of blood leaked out from the cut, down his arm, to his shoulder and staining a small patch on the corner of his sleeve. Once she was lifted high enough, Bill applied more pressure and shifted his feet before kicking at her stomach violently; she was pushed off of him and he rolled on top of her instead; using Dipper's weight to hold her down.

Dipper himself felt a captive in his own body; forced to watch as Bill clawed at the acheri below him. 'Bill!' he screamed out in his mind, 'Stop!' The demon paused briefly at Dipper's command, seemingly having forgotten about his presence.

Just as Bill was about to ignore the human's pleas in favour of digging his real, physical hands into the being below him, a hand was gripping his arm and pulling him along. He followed the teenage girl who was dragging him away, the smile was purged from his – Dipper's - face by the blood lust that roared and demanded to be let out. He chose to ignore his own want for once and instead ran faster; Bill and Mabel ran all the way back to the Mystery Shack, glancing back every so often to be sure that they weren't being followed. Once they had exited the thick crowd of trees, they stopped running, both panting from exhaustion, albeit Mabel more than the immortal demon. Bill took note of the scratches and cuts that Dipper's body had obtained while in the woods; he thread a sliver of his magic through the boy's veins and let each wound heal fully. Once done, he reluctantly gave control of the vessel back to the teen, who sighed gratefully at his regained senses. The demon appeared in front of the twins; "well, see y' tomorrow, Pine Tree!" Bill said happily, tipping his hat to the boy, "Shooting star," he then added less happily. He disappeared in a flash of gold without another word.

A sudden pain shot through Dipper's arm as his sister punched him; he flinched and rubbed at his arm and complained: "ow, what was that for?"
"I should be asking you that! What was going on back there?" Mabel asked, concern and a hint of anger spreading across her features.
"What?"
"You and that ackri or whatever it was! You were acting all crazy!"
Dipper paled slightly; "that was Bill, not me," he defended.
"What?" This time it was Mabel who asked.
"It was part of the deal, he'd control me if it got dangerous and get me out of there safely," he explained. "He did, too. See?" he lifted his arms and turned around slowly to show not a scratch on his body.
"Dipper… I don't like this." Mabel said.
"Mabel there's nothing wrong. Since when do you care anyway? You've done plenty of things just like this; remember Mermando and the gnomes?"
"But they weren't demons, Dipper!" Mabel argued. "Bill's really… weird. I don't like him."
"Alright fine, I see your point, but… he hasn't done anything yet, right? He's helped and we both made it out without a scratch on us." Dipper failed to notice Mabel rubbing at her arm over her sweater. "Tell you what," he said, "if he does anything bad that isn't part of the deal, I'll call it off, alright?" he proposed.
Mabel hesitated for a few moments before nodding her agreement; "ok" she said.

Dipper checked his watch; "it's only two thirty. I'm going to finish writing about the acheri, after that do you want to go to the arcade?"
"Sure," Mabel agreed, beaming.
"Bet you can't beat me at a game of Fight Fighters," Dipper challenged.
"Bet you I can!" Mabel accepted the challenge.

If you're interested in reading about the acheri (a real myth) then: wiki/Acheri
Don't worry it's only short.

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