Long story short.
No one liked Sombra when he was a colt.
He was a orphan that never got adopted.
He finds out he was most likely destined to become a smoke monster.
His best and only friend was leaving him.
Finds out he's actually part of a fairy smoke monster race.
Goes coo coo for coco puffs and turns the empress of the empire to stone and breaks her.
Takes over the empire.
Celestia and Luna beats him.
And you know the rest of the story.

You really gotta feel sorry for Sombra. Not because of his backstory or anything, but because the writers just can't seem to do him justice. He does almost nothing in his debut episodes and then promptly gets killed off, the less said about Reflections the better, and now all this mess happens. The Season 5 finale actually managed to portray him as a credible threat, but for all of what, five minutes?