You need an arm. Need it.
Making it would take time that you don't have, fitted away in between fitful naps, licking blood up off the rocks, scrabbling in the dirt for worms, and running away from Dummy.
You need to take an arm, or something already in the right shape.
The trick is just in finding it.
You don't know how long passes before you find it. It could be anywhere from weeks to months, blurring together in a haze of just managing to survive, as you're forced to bypass options, or are denied them.
The big monsters are often just too big. Even if you could bring one down, their arm would be ungainly and clumsy. Not useless for your purposes, but too much effort for too little result.
The spider woman? That one has plenty of arms to spare, and they're at leas sometimes of close to the right shape and size. Enough that you can grow into them, anyway. But the trouble there is that you doubt she'll hand one over willingly, and venturing through her cratered home... the whole area is a trap comprised of smaller traps which link together to funnel victims toward the ultimate end.
You spent a couple hours observing it from a distance, but then moved on. The spider woman could pass through her own traps in a way that nothing else could, and without that ability the whole place didn't have any openings large enough to pass more than a fist through without starting to accumulate the silken threads that spelled doom.
You might be able to move through. If you had that spider woman's ability. If you could fly. Or if you sent something much bigger through first, to clear a path. With none of those, the only solution that comes to mind is to burn it all, and the resulting conflagration wouldn't be at all useful for finding a suitable replacement arm.
You considered the amanojaku for a time.
Smaller, oni-like figures. They seem to grow bored easily, and spend any time not trying to sniff you out and eat you in taunting and attempting to trick larger and more dangerous things into fighting each other.
Sometimes it works, which prompts possibilities for the future. But the times it doesnt strike those possibilities clean through. The broken and ruined bodies of amanojaku just translate in your head too easily to broken and ruined bodies of Mio.
It might have been a convenient way to snap up an arm, but considering the monsters around here seem to have no compunctions about eating one another, there's usually very little left. Definitely not a full replacement arm. Since bits are sometimes left behind, it might have been a possible avenue, if you could snatch a corpse before the bigger demons started eating, but....
Then you found the corpse.
-
Chained up and in sealing wards that appear to do nothing, it seems to be exactly what you need. One dinged and slightly battered arm outstretched from a withered and contained corpse.
It looks in good condition. You mean, comparatively. It's just what you need. In a blur of mental stress and fatigue, you put one foot against the corpse's face, grab hold of the wrist with your remaining hand, and pull.
It wrenches off easily, though you belatedly realize that it won't do at all. Not only does it separate below the elbow, it's the wrong arm entire-...
You blink and rub your eyes.
Correct arm now. Ending at the right place despite... yeah, despite that the chained up corpse still has both elbows of its own.
"Well, this idea just went from okay-ish to probably terrible, but all right then." you mutter to yourself.
Then you jam the arm to your partially healed stump.
... You don't even need to do much after that, as though the limb was just waiting for the opportunity. It's excruciatingly painful, but it seems to be driving tendrils back in your flesh every bit as much as you're streaming real and fake chakra coils into it. The pain is joined by a sensation of befuddlement.
Then that's replaced with the urge to go find the nearest thing, and kill it.
"Listen. I am way ahead of you there." you say aloud. "Drop in the bucket, buddy."
The mental pressure redoubles, tendrils of force and will seeming to sink into your brain.
You mentally set them on fire. Which. It... seems to work, which you weren't entirely expecting but at the same time aren't completely surprised by either. Chakra's components are physical and mental energy, right? And you've got, for various reasons, more chakra than average for a passable academy student of your age.
There's a sensation of confusion and outrage demanding your submission and you're pretty sure now that you've grafted some kind of demon world parasite onto yourself. Arm's working like a perfectly normal arm, if colored and with the sheen of volcanic glass, but there's mental contamination.
"Nah." you reply aloud.
And then Dummy crashes down from the sky like a fucking meteor, club first, and your will and the parasites becomes united in the decision to be anywhere else, quickly.
... It takes you a couple of days to give Dummy the slip this time. He's getting persistent.
Now then.
---
[ ] You need to find better sustenance than blood licking and worm sucking.
[ ] You need to find better shelter than tree branches and nothing.
[ ] You need to practice using your arm for ninja-ing. Get the kinks out.
[ ] You need to beat the will behind your new arm into submission, somehow.