These are just very basic corrections. I apologize ahead of time if they seem a bit harsh, it's not intentional, its just that I'm being thorough, and in all truthfulness, a lot of changes were needed to bring this up to my basic standards.
As I lay (incorrect tense, the rest of the whole chapter is in past tense) on the top of the hill, I can feel the warmth as it surrounds me (incorrect tense, missing a word)with a veil (incorrect spelling) of comfort. The sun rose from beneath (personal opinion, but 'beneath' sounds better than 'under' to me) the horizon as its lights ('it's', needs to be possessive not plural and 'light's' is the opposite) flashed what had seemed like an elegant painting in the sky. Orange, yellow and pink roared over me (shouldn't have been capitalized, and I get that you're trying to put some imagery into this, but I don't think roar is very appropriate considering colors are associated with sight, and roar is associated with sound). I felt not a care in the world at all. Just my friends and me, (contrary to popular belief, 'I' is not always correct in this instance, this should be 'me') traveling the vast green land, no longer having to worry about ever going home. I was free.
Until one day, the very horihad (I'm not sure what you were trying to write here, it seems to have been accidentally cut off) betrayed me, and those colors (colour is correct British spelling, but the apostrophe 's' is still wrong. I changed it anyway, though) I had enjoyed for the longest time had been stolen from me. I felt (flow problems and indirect word usage) as if someone pierced my heart and then ripped it out of my chest. What was happening? (this could be italicized, but it's a personal decision, I've seen it done both ways) I asked myself. The sky had turned completely black. It was still and heavy, I couldn't see a thing.
I heard my pokeballs (no apostrophe as it's plural, not possessive) as they rattled (adding 'from' is filler and not grammatically correct, also makes more sense to have an action related to sound, since the character heard something earlier in the sentence) inside my bag. Closing my eyes (again, plural, not possessive, first word changed for sentence variety purposes), I imagined my friends: Umbreon, Espeon, Pidgeot, Blazekin. (I like colons, I approve!) Do I dare open my eyes? Or do I keep them shut, so I could continue to extract comfort from the strength (flow problems, 'further' is a word measuring length, not time, also misspelling of strength) and bravery of my friends?
No, I couldn't (this change is more a personal preference). This wasn't something I could just close my eyes to (again, a plural. And you close your eyes 'to' things, not 'on' things unless they are inside your eyelids, O_O), and make disappear (more flow issues, I just think this sounds way better).
So I opened them and looked up, but (makes more sense, also spelling should have been 'though' if kept) I couldn't see a thing. My eyes (plural, not possessive) wouldn't adjust, what was going on? (this seems a more likely question to go between the others, also more pertinent) Was I dead? Or would (incorrect tense) I wake up from this nightmare?
And there I saw, if I saw anything in that tragic, dark place which seemed like the bottom of an abyss, what appeared to be the cause. (This is the best I can do with this sentence without changing the whole thing. It's hard to pin down what exactly is messing you up here, its just a lot of little things) It wasn't a friend, it was something entirely different. (Eh? Why would he think it might be a friend?) Its one blue eye lit up the entire sky for an instant and then vanished, releasing my heart and the colors up above. Everything was back to normal, but I didn't know for how long.
I began to tremble, just like my pokeballs. (I moved this sentence, it seemed out of place where it was.) I decided to run back down the hill to Alvenu Town, which was a small area I had last been. I ran, and I didn't stop until I had reached a Pokemon Center.
There I asked one of the nurses, "Did any of you see what just happened? Th--the sky..." (Especially with current action and dialogue you should use the past tense 'I asked' rather than 'I had asked'. The latter is more for telling about the past while already in the past. Also, the two hyphen's should technically be an M-dash. In a word processor you could insert one easily but I don't know the key-code for it and I assume you don't either. I'll say no big deal on it.)
The nurse looked at me as if I had four arms and two heads. She said that no such thing that I had described to her. (All the rest of the sentence was confusing and uneeded. I would suggest giving the nurse actual dialogue though, instead of telling us what she said, have her actually say it.)
Alvenu Town was famous for its library, so that's exactly where I went next (more extra, unneeded words deleted). What I saw had to be a Pokemon of some kind, so I decided ('it was my intention' sort of implies he intended to do something but never really did, which isn't the case) to find out which Pokemon it was and how I could deal with it if it were to ever return.
It must have wanted something from me personally(clarification) if the people down at the Pokemon Center claimed to see no change in the sky, I even got the same looks from the people down at the library. I went into the back, thinking to myself that (sentence variety and flow) I would probably have better luck looking for something mysterious and dark. The book I chose from the shelf was entitled Pitch Black, (references to books are italicized). I'd had to translate it since it was spelled through the Pokemon, 'Unknown'.
I flipped through the pages, and found the entire book had been written in the same script the cover. I grew more and more confused, anxious and eager (spelling), until I flipped to the very last page in the back of the book.
It was a picture, and under it was the written, " ダークライ". Darkrai.