Oh wow... This actually looks like it could happen. It's so sad. Poor Trixie... and Luna, Get 'em!
The drawing for this is really good and You can feel the emotions in the picture. Also, you captured the battle between two very shadowy beings well. Luna vs Sombra is just a fanbrony's dream.
As for the idea that it's just head canon, it's still awesome and you should do more like this. Do an entire storyline because I think it would be epic.
Oh luna, defend Trixie and she will learn that she is not everything and more. Great and Powerful needs to be humbled once in a while.