Chapter Text
It starts in his teeth.
It’s a tingle at first, nothing more than a passing toothache, nothing Izuku really thinks anything of. He avoids cold foods and drinks for a few days, hoping that it will go away on its own, and when it doesn’t he goes to Recovery Girl for a quick check-up. Nothing is wrong, but she tells him to make sure he’s brushing and flossing, and to keep avoiding cold things for the time being.
The ache doesn’t go away; it gets worse.
It goes from a slight ache, to a throbbing pain, to a shooting agony, spreading from his teeth to his gums to his whole jaw. He goes to Recovery Girl again, and this time when she looks at his teeth, she freezes for a moment in surprise.
His teeth are becoming fangs.
She doesn’t know why, can only speculate that it might have something to with One For All, or even some latent quirk factor of his own, and give him painkillers for if the pain persists.
They can only let things take their course.
Next, it’s in his fingers.
It feels like he’s bruised his nails, and then like they’ve been ripped off, and then like pain is shooting up his tendons, all the way up his forearms. He’s struggling to write, to hold chopsticks, to do anything that involves more than clenching and unclenching his fingers in pain.
His nails turn black and harden, grow faster than he thinks is natural, sharpen to a point no matter how much he tries to file them. They thicken and solidify to the point he can no longer file them down, and now he has claws.
With his new claws comes the instinct to claw.
Next, it’s his eyes.
It begins with itching, like constant dust or pollen getting in and irritating them, and no matter how much he washes them, how many allergy tablets he takes, nothing helps.
He has to remind himself not to rub his eyes, because now he would just gouge them out.
The itching eventually becomes a throbbing pain, the kind that he knows from looking at a screen for far too long, but deeper, like it’s reached all the way into his eyeballs and his brain and squeezed. Then it hurts so much he feels like he’s going to go blind, like gouging out his eyes might not be so bad, and by the time it’s passed he finds himself waking up in Recovery Girl’s office, hands in mittens to stop him from clawing at himself.
When he looks in the mirror, his sclera are black, and his irises an unnatural, glowing green.
Next is his back.
Despite all the recent changes, he really doesn’t think anything of it at first. He spends a lot of time sitting at a desk, and when he’s not there, he’s fighting and pushing his body beyond its limits. He thinks it’s just normal back pain, and that it will be gone if he just makes sure to stretch more until it’s better.
When it doesn’t get better, he realises what might be happening and braces for the worst.
The ache becomes a stabbing, becomes a tearing as he feels like something is breaking through his skin, and then he realises that something is breaking through, that on each notch of his spine, a spike now protrudes, catching on his t-shirt and tearing tiny holes in it.
When he manages to see them in the mirror, they look just like his claws; black and shiny, like raw obsidian.
Next is his lower back.
This time he knows it’s something else, something new, and braces for the pain. It feels like a bruise, then a sprain, then a broken bone, a bone-deep ache spreading to his lower back, and finally something snaps and he passes out, coming to once more in Recovery Girl’s office, with something else he didn’t have before.
This is the biggest change so far.
It’s a tail.
It starts at the base of his spine, long and straight with a point at the end, going from dark green at the base to pitch black at the tip. It swishes idly when he isn’t thinking, jerks and twitches in reaction to his emotions, and the sharp point tears fabric and paper and even a wall.
He can’t control it at all, because he’s never had one before, and it keeps on knocking things over, whacking people in the legs, leaving scrapes and bruises.
He talks to Ojirou, who helps a little, but his own tail is not the same, and so Izuku is left with a new limb he can only sometimes control, a lot of questions, and very few answers.
Finally, it’s his temples.
It’s not quite his temples really, more like the top of his forehead, just along his hairline, and not somewhere he’s ever really had a headache before. It wasn’t even really like a headache, more like a swollen bruise just beneath his skin, becoming another pushing, another breaking through, until he is left staring at himself in the mirror, two streams of blood streaking down from his forehead and past his eyes like tears.
Horns. He has horns.
They’re not too long, a little less than the length of his hand, and have the same colouring as his new tail. They’re the same obsidian-shiny material as his claws and spikes, and just as sharp, curving slightly outwards before pointing straight up.
He no longer recognises himself.
Midoriya Izuku has become a demon.