What if the reason we don't know much about King Sombras past, is because he himself doesn't know about his past. What if the Dark Side made him forget?

4194724 We don't know much, because the show does little effort into fleshing out the full history of its own universe in detail.

4194724 The show doesn't really do anything to describe him. Only giving us, he's a threat, did something we didn't like, boom BAD GUY.

Well, we're about to get his backstory regardless. April 1st, FIENDship is Magic #1 comes out. Sombra's backstory in comic form.

I'm actually writing a story where Sombra has to reclaim his past now. It's coming out as soon as I finish my current story. Though the story isn't solely about Sombra, and since I'm already well into development I'll be ignoring the Sombra comic coming out. Still definitely going to read it though.