Okay, so, yes, I did leave a comment, and I love you as an artist, so I will leave a critique for you.
I absolutely love your brush strokes. Her mane is very well done, the darkened tones really add to the emotion of the picture. The darkened world around her feel like a bit of insight to what is going through Celestia's head as she is casting the dark spell working through her body and horn. What falls short for me is her facial expression. I don't know what exactly the story is behind your inspiration for this piece, but her face leaves me feeling like she is tired. It looks like bags under her eyes and a frown rather than hatred in her eyes and a scowl that I would expect from watching that moment in the show. I would love to see more of the darker emotions in your works, but if this was meant to be Celestia casting darkness, there is a little that you need to keep an eye on with the eyes if you want to make the anger the primary focus of the piece.
Overall, the piece does leave me with chills, even if there is going to be Evilestia paintings by the dozens within the next few days, I have no doubt that yours will be among the top 3-5% for quality of the work, very well done.