Twilight had failed. She had found the Elements of Harmony, but she had not been fast enough. Before she could reach them Nightmare Moon got her hooves on them, and she destroyed them right in front of her. She is sentenced to death by Nightmare for threats against the Crown.
Comet, a fairly young colt with a knack for Magic, especially dark magic, has for long have had feelings for Twilight. Now with her execution on the horizon, he think that he can save her from death with a spell he devised that combines Necromancy, Blood Magic, and a few other arts. But will it be enough? And at what cost?
Revisiting because, let's be honest, sometimes I'm blind... 
If something seems wrong, or doesn't fit, or a word is missing, please tell me and I will fix it as soon as possible.
Not bad. By the looks of it, it seems you have revised this OC so that they are no longer an alicorn. Smart choice, if I do say so myself.
Let's dive into it a bit:
+: Well-made opening summary with beautiful formatting, as well as a fairly decent introduction to start us off.
- / + : Color scheme of the OC does feel like it clashes a bit, and the red eyes do seem a bit stereotypical; his name really doesn't fit in my opinion. On the other hand, his has quite the unique talent and is very mature even for his young age, which might act a little as a deficit; it felt as if you were writing a rather mature adolescent teenager rather than a 10-year-old colt.
- / + : Young Twilight and Celestia feel a little out of character, including the nobility trying to punish him. I can't imagine Twilight so readily willing to plead for the sparing of his life, nor see why she would be in Celestia's court in the first place. Celestia is also the sole leader of Equestria at that time, and shouldn't be so ready to submit to the pesky nobles. On the other hand, you recovered by reverberating the punishment so that it was actually to his benefit, so points for that.
- : Continuity errors here and there; if there was a law that punished those who practiced dark magic, how did the nobles know it even existed? How did the nobles even know he was practicing dark magic? Unless it was particularly strong ( which it didn't sound like it), then they shouldn't have been able to distinguish the difference between Dark and Harmonious Magic.
- : Pacing felt a little too rushed. The ending to this felt very crammed, If you had gone on to explain more of his experiences with Twilight to further enforce the bond between then, then I would have felt more sympathy for him nearing the climax; I cannot express much as it stands. It could have easily been a 6,000 word chapter if not longer.
- : When there was dialogue, it felt like too much exposition was poured in at once. Large blocks of text filled up most of the trial with Celestia and Comet, which made it a bit jarring to read. Separating it with actions such as sobbing and small breaks in between seem a little more human, and less robotic in the process. Don't be like Pinkie Pie and run on with your sentences.
I give it a 6/10 so far. It may seem a little harsh, but a lot of this can be readily and easily fixed through a little editing. It looks like you got a good story on your hands.
7073453 Uhh, his eyes... Oh, that's meant to be bloodshot not blood red... His eyes are actually orange.
7073597
Amber eyes are beautiful. You should describe them in the story.
7073620 Edit: Did it now also fixed the parts that were unchanged. I managed to press refresh instead of save while I was writing, it was in the middle of the night and I was too tired to really notice the strange wordings, most things should be fixed by now, again, thanks for mentioning that.
Whatever happened, i did not get it.
There is simply no logic in it.
No explainable sense.
Just an ass-tonn of hot air and over-dramatic cliche-s.