Here's a proofread version of your story:
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After another uneventful night, the light of dawn began to emit its faint radiance, and the sun rose from the east beyond the mountains as it always had. The birds awakened the world with their harmonizing songs, and a cool breeze rustled through the leaves of the trees.
This was the scene he always enjoyed in the morning, watching from the confines of his prison—a prison in the middle of a forest surrounded by mountains.
It may not be entirely accurate to describe a big concrete wall with shackles in the middle of a forest as a prison, but the angel's condition and emotions suggested otherwise.
A few hours after he woke up, there was a rustle from the bushes nearby, and a little girl emerged, her eyes full of innocence.
"Morning, sunshine," said the prisoner, his pure white wings glistening on his back.
"Good morning, Uncle Angel. I brought you an apple like you wanted! Now tell me your story as you promised," the smiling little girl replied, handing him the apple.
"Haha, straight to the point, huh? Okay, okay. Now, let's see. Where should I start? Hmm, I guess from there is the best choice," said the prisoner, taking a bite of the apple.
"Gulp. Ah~ it's been a while since I ate something this sweet."
"Hehe. Uncle Angel, I bought it from the village's best orchard!"
"I like it, thank you. Now let's start the story. It all began when I woke up with a really bad headache…"
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"A-argh... my head..."
An unconscious young man lying on the ground awakens with a head-splitting headache. Struggling to sit up, he waits a few moments for the pain to lessen.
"My head... What happened? How did I get here?... WHO AM I?!" With his hands on his head, he suddenly panics, suffering from amnesia. Unfortunately, no one around can answer him, so it takes a while for him to calm down.
"O-ok... No worries. You've just woken up from a coma, not knowing a thing. That's bad, but you can get through this... You could get help from a hospital. Maybe they can scan my brain or something," he encourages himself, his mind surprisingly rational in this bizarre situation.
"Where am I? Hmm, looks like I'm on some kind of hill and..." The young man, trying to ascertain his whereabouts, looks up and suddenly falls back down onto his rear, his eyes wide with fear as he takes in the frightening scene above him: a huge, gaping hole in the sky.
"How is that even possible?" He doubts his eyes as he stares at the hole; the laws of physics shouldn't allow such a thing.
"Oh my God..." The sheer shock forces him to mutter this single sentence as he takes in the bizarre sky.
It was bright gray with a hint of purple, and what looked like runes and symbols were carved into it. The red runes, like fresh blood, formed lines that crossed each other every few meters, creating a never-ending grid that abruptly disconnected at the hole.
"Am I dead? Is this hell? No, I'm not being tortured or anything. Plus, I like to think I was a good guy," he reassures himself sarcastically, trying to lift his spirits, and then sighs in contemplation of his situation.
A swift breeze makes him shiver. "Brrr, it's a bit cold. Why am I wearing a T-shirt in this weather?" He looks down at his black pants, white T-shirt, and black shoes with white socks.
"Okay, let's check what I have on me... Hmm? What's this?" Searching his clothes, he finds a piece of paper with scribbles in his right pocket.
(You are Leon)
"What terrible handwriting… Well, now I know my name. But what about this place and how did I get here?" Leon thinks while flipping the paper. "Ah! There's more writing on the back!"
(Look around you until you see a massive mountain. The mountain is due west from where you are. Go to the west through the forest. Once you enter the forest, count to 60 and trace your steps backwards until you exit from where you entered. You will arrive at a shack with a week's worth of food and water. P.S. Please follow the instructions to the letter, or you will be lost.)
"..."
"That's super weird... telling me to enter a forest only to go backwards immediately." Leon wishes for more information—at least something more helpful than this. "I literally have nothing to lose at least…" He can't stay in place; he'll starve if he doesn't find food.
Looking at the mountain, he has to admit its massive size. He turns in the opposite direction and begins walking until he arrives at a forest with gray wooden bark and red leaves. As he enters the forest, he feels a deafening sensation, like when someone dives into deep water or reaches outer space.
Creeped out by the silence, he starts talking to himself to break the eerie atmosphere. "What a weird place this is, with strange trees, no wildlife or ambient sounds, not even the wind... and that sky." (Leon glances at the purplish-gray sky.) "It's so unrealistic—like I'm stuck in a dream, the dream of a mental patient... Maybe I'm the mental patient in my own dream, and the outside world is fine... or maybe not; it could very well be a case of fever dreaming…"
"I feel down all of a sudden because of these dark thoughts. Note to self: do not let yourself get immersed in your own thoughts; otherwise, madness and paranoia are possible outcomes, especially in these strange circumstances." He frowns. "Also, that thought of being stuck in someone's dream seems a bit familiar... Where is that mark or sign? I counted more than sixty. Did I stray from the path? No, that's unlikely. What about the back..." Leon stops speaking, speechless, as he sees the hill he came from right behind him.
"What the... No way, it can't be..." Going back through the forest entrance, he sees something other than the hill.
"A-a shack! But I swear I didn't see a shack the first time I descended that hill. How and why did it appear when it was an empty hill literally just a minute ago!? It's probably this strange land's doing. I've got to leave quickly! I hope what is written on the note is true." Leon walks briskly to the shack, not wasting time. He immediately knocks on the door.
"Hello, is anyone inside? I just got here and... oh! I forgot to introduce myself. I'm Leon and... well, I'm in trouble! I just woke up and... hello?"
Frantically calling to see if anyone's inside, he finds only silence answering him. The shack looks like it has seen better days. "Could it be abandoned? I'll knock again just in case." Just before Leon's hand lands on the door, it opens slightly.
"Um... the door is open. I'm coming in! I hope I'm not intruding... and… yup, no one lives here. Well, figures, since no one would want to live in this weird land. Let's take a look around." Leon enters the abandoned shack, looking for clues about this land or at least some shelter.
Inside, there's a wooden table with a chair by what looks like a small framed window to his right, a trunk with a metal frame and wooden boards on the left side, and a mattress with a ragged blanket—no pillow in sight. Surprisingly, the inside of the shack is quite clean.
A sheet of paper lies on the table, and noticing it, Leon immediately picks it up and starts reading.
"This shack will be your shelter for the time being..."
"This handwriting... it's the same as the one on the memo I have. Could it be that someone left these for me, knowing I'd go through this situation? But how would they know? There are too many variables. What if I didn't follow the instructions? Did they know me by heart and can actually guide me through all this? Whoever this is, they should know my past. Hmm... that might be a way to restore my memory and find out what happened to me. But ugh, it's a long stretch, I know. What have I got to lose?"
"You should have noticed the metal trunk at the back. In it, there's survival gear and the food rations I told you about in the first memo. Now for information about this land..."
"Oh boy, here it comes..." Not even checking the equipment and rations in the box, Leon continues reading anxiously.