(read on ao3)
Light finished sewing the secret compartment into his wallet and showed Ryuk how he could slip the Death Note inside.
"You're pretty skilled with your hands," Ryuk said. "I bet you're pretty popular with the ladies."
Light smiled as he sat back in his chair. "Skill has nothing to do with it. It's your looks that count. I'm guessing you're not that popular, are you."
"Huh?" Ryuk said.
Light was pulling out his phone when Ryuk continued, "I guess this skin does look a little awkward to you humans. But you can't judge me on that, since it's not even what I really look like."
Light paused, flipped his phone closed, and looked over at Ryuk, who grinned at him meaningfully, his long, yellowed fangs glinting sharply inside his wide-stretched blue mouth.
"Don't tell me you're in some kind of human disguise," Light said with disbelieving condescension.
"Oh, but I am."
Light knew it would probably be better to leave it. If Ryuk was telling the truth, there was probably a good reason for why he was going to the trouble of wearing a disguise. And he looked just a little too excited for this to mean anything good for Light. On the other hand… "So what do you actually look like?" Light asked, unable to stifle his curiosity.
"Wanna see?" Ryuk said. "I can show you." He moved into a sitting position, folded on the side of Light's bed, his too-long limbs crunched in the small space. His long, skeletal hand hovered at his collar, and then he… un-zipped himself.
There was really no other word for it.
It was as though both sides of his flesh just pulled apart as he drew down his claw. Light caught a glimpse of white glimmers of fat, and the two sides fell as though there were no muscles and no skeletal system at all to support them. For a moment, that's what Light almost expected to see, frozen in horror: a glistening red thing like the mannequin in science class measuring the muscles, or the empty skeleton underneath, but neither appeared. Instead, the floppy skin just stayed open, from Ryuk's collar to where the bottom of his sternum should have been, and instead of ribs and a heart…
There was something under there.
A vast thing that Light saw as though he was looking through a telescope's lens. It went on and on in rotten patches: the bright blue and purple and green and grey colors of mold, folded up and crinkled like mountainous slopes. If he'd stepped forward, he felt certain he could easily step into it by accident, and then be lost. Along with it came a chill that reminded him of the morgue in the police station, where he had wandered into alone as a boy. It had that same emptiness to it, but there was a choking haze in the air and in his nose that seemed like dust—or spores… no, Light realized, as he saw it shifting, those endless mounds rippling into the distance, moving as though there were creatures scuttling underneath—no, it was sand. Sand made out of dead things, ground shells and bones from the bottom of long-dried oceans.
The vastness and the horror of it was too much to fit in the entire room, but Light thought later that if it had only been that maybe he wouldn't have screamed.
Because he was screaming. In terror, with every ounce of breath in his lungs, but even the sound of his screaming barely reached his ears, and even though a distant part of his mind told him to stop—he couldn't. To stop screaming, he would have to remember how to work his own jaw, how to breathe. But instead he seemed stuck, unable to do anything but look.
There was a thing reaching from the crumpled sand-mountains. It pierced its way through the skin-disguise as though it had been paper; a long rope—no, a cable—no, something more alive, or rather… dead… a mass of sinews or something like a bent arm, spiked at intervals like thorns on a stem with thick, obsidian darts. It drifted and coiled and moved as though floating in the air, like it weighed nothing, but it was thicker than Light's arm. And, even though at first Light seemed to see the whole length of it, the more he looked, the longer he realized it was. Somehow it all managed to coil around them without filling the entire room, though it seemed to fold the air itself into impossible origami shapes to manage it. And the end of this claw—spine—thing… sunk its way deep into the left side of Light's chest.
Through the skin.
Into the heart.
There was a sucker-type end, a round-ish blob of some type, and in a ring around the edges of it came needle-point nails. Teeth?
They sank down through Light's body so deep it should have drawn blood, but there was no blood. The skin around the sucker, though, was greyish and spongy, as though it was rotting away from the spot of penetration.
The rest of Light's body, except for one hand, looked ordinary. But the hand he used the most to write in the Death Note, his right, was rotted up to the elbow. He could still see the skin, but it was mottled with spots like mold, greyish patches and whitish patches, and it was sagging away from the bone which was visible in places. The fingers on that hand were sticky, tacky, dripping with a mass that smelled vaguely like blood, but moved like part of his own skin and hand, if his hand had turned into webs. And the slimy mess of it, that smaller cord, went over to the Death Note, until at the end of it it had turned into pages, and Light became sure in that moment that the pages and the cord and the rotting hand were all part of one thing.
Someone was pounding at his locked door. "Light?" his mother called. "Light, honey… what's going on? Light!"
"You'd better open that door," Ryuk said, "or your mother's gonna be worried." He zipped himself up, and all of it: the thing inside the skin, the cords, and the rottenness on Light's own body disappeared.
He was lying on the floor, his throat raw, curled into a ball.
And then he remembered how to stop screaming.
When Light unlocked the door and opened it, his mother thought he looked a little shaken, perhaps, but he managed a fairly convincing smile that seemed at odds with the bloodcurdling shrieks of terror that had sent her running up the stairs two steps at a time, sure that her only son was being murdered, or worse. Or worse. Sachiko didn't know what could be worse than that, but during the fragmented moment as she'd ran up the stairs she'd thought it to herself: that if she burst through that door and found someone carving up her son's body it would be a kinder fate. Something she could understand.
Of course, now, she couldn't believe such a weird, wild thought had come to her. And so she let it slip from her mind, and felt reassured that Light was in front of her and seemingly unharmed, and she didn't look too closely at his eyes, but only stepped forward and hugged him tightly.
After a moment, Light reached up and hugged her back.
"I'm okay, mom," he said shakily. "I was watching a scary movie. I guess it got to me more than I thought it would. I won't do that again."
"I'm just glad you're okay," Sachiko said. She stepped back then, and smiled at Light awkwardly, feeling silly. Only a scary movie, and surely her mind had been running away with her. Mothers! She laughed lightly, and was secretly glad that Sayu was at a friend's house. She felt that Light ought to need comforting, but he was standing easily in her presence, a little removed. Grown up, she always thought. He wasn't her little boy anymore. But these things happened.
Even still, perhaps she should sit with him awhile and talk. It had been so long since she'd gotten a chance to talk to him about what he was thinking and feeling, about anything but his schoolwork.
But the room was very chill and forbidding. In fact, something about it seemed to make her feel distinctly unnecessary, as though she were intruding; even though Light was still smiling at her.
The feeling of intrusion grew unbearable, and at last she stepped back over the threshold, and said something inane, taking refuge in the familiarity. "I can bring a snack up for you later if you want."
"That would be great. Thanks, mom," Light said. He closed the door.
And turned around to face Ryuk. The burning glare seemed to crackle through the air between them, and Ryuk felt a small moment of 'perhaps I shouldn't have done that' that he did not quite recognize as guilt, having never felt anything of the sort before.
But it was enough to make him sheepish.
"Ah, er," he said, scratching the back of his head, "I guess that's why we don't tend to open ourselves up to humans. There wasn't a rule about it, so I figured what was the harm… heh."
"Ryuk," Light said. He took a deep, composed breath, and Ryuk moved gingerly away from him, because Light looked like he was about to explode any second, and Ryuk was pretty proud of his self-preservation instincts. But with three more breaths, Light was finally able to say, with only a scathing edge to his voice, "what was that?"
"Me?" Ryuk said.
"I mean the, thing. The… part," Light said, gesturing between them, "that was stuck in my fucking heart!" He was shouting. He stopped, and breathed heavily in the silence, and Ryuk glanced longingly over at the basket of apples at Light's death, and decided it would probably be better to wait.
"I guess you'd call it a proboscis," Ryuk said.
"I'd call it a proboscis," Light echoed flatly. "Are you feeding off me?"
"Well," Ryuk said. "...Kinda?"
Light started to scowl, and Ryuk hastily clarified, "not in a weird way! Look, you know shinigami aren't allowed on Earth unless they're either about to kill someone, or they're possessing a human with a Death Note. Right? Ever wondered why that is? It's 'cause… the human world… it's the world of Life. If shinigami were to go down to the human world without doing either of those things, the world of Life would recognize us as a foreign body—something that shouldn't be in its system—and attack us. It wouldn't be pretty. But, if a human picks up a Death Note and becomes the owner of it, a shinigami can possess that human, and 'use' their lifespan to trick the world of Life into thinking we belong here. Basically," Ryuk finished, "as long as I possess you, the human world thinks I'm you, and I'm safe. The proboscis is pretty long, so I could go up to 14 kilometers away from you while still being attached."
"Like a parasite," Light said thoughtfully. The explanation seemed to have calmed him down. A new piece of information always piqued Light's curiosity enough to almost forget what he had been feeling before, Ryuk had quickly realized about his human. And, as long as Light found a way to justify it, he could get used to anything.
Other humans, explanation or no, would probably still be screaming in horror and trying to claw out their whole chest.
Light really was pretty impressive.
"Okay, that makes sense," Light finally said. "But what about the stuff connecting my hand and the Death Note?"
"Well, that's also about the bond," Ryuk said. "When you claimed ownership of the Death Note, you put a little part of the world of Death into yourself. And not just any part of the world of Death, but (because it was a Death Note I dropped) a little bit of me. That's why I can possess you in the first place, why your body doesn't reject me like the rest of the world of Life does."
Light took this information in with a mildly shocked and disgusted look on his face, and glanced over at the Death Note in perturbation. He wiped his right hand on his slacks, as though he could somehow get rid of the memory of that slime connecting it to him.
"I see," he said at last. "Ryuk… just for future reference?" he said with a sigh. "This falls under the heading of 'things you should have told me in advance.'"
(on ao3)