Very good drawing. I would like to use this picture for a fanfic I am writing. It will be recognized as your work, and I will make no changes to said work. I will not use it until/if you approve. Thank you
In my headcanon, Sombra was a past student of Celestia as well, and was a unicorn prodigy like Twilight. Unlike Twilight, he let dark magic consume him, and it turned him into a power-hungry tyrant. By the time he took over the Crystal Empire, his mind was too warped by dark magic and lust for power and control to be brought back to the good side, so Celestia and Luna had no choice by to seal him away.
Part of me thinks of him as either as you say of student of Celestia or Luna. I feel that he was sent, like Twilight, as an emissary to the Crystal Ponies as a test to further his studies. Upon discovering the riches they had, Crystals everywhere,his own power, only unicorn among earth ponies and the aforementioned Dark Magic twisting him to evil prompting him to conquer the land for himself, forcing his former teachers to put down their wayward student but not before making the Empire disappear to spite them and show how powerful he had grown even in defeat.
I really like him as a character. The right mix of powerful and intimidating menace with plenty of open space to fit any theories. I'm so fighting the temptation to write a fanfic.
xP Appearances are deceiving. Celestia and Luna are at least two thousand years old, and they look pathetically young. We have no reason to believe Sombra is immortal, he's not a spirit like Discord, nor an alicorn like Celestia or Luna, he was just sealed away as a shadow for a thousand years, which is why he's still around.
No other explanation? Sure there is xD It's called dark magic is a field of study and has been for as long as magic has been around. Being the princess, she needs to be adept in using all kinds of magic, dark magic included, if need be. Sombra wasn't taught it by Celestia in my view, he did it himself in secret, knowing she'd never teach him.