Twilight wants to know what happened to Starswirl the Bearded, but nopony who knows will speak. How many wounds is Twilight ready to reopen to get her answer?
In a string of theories, I concurred this one while working late at night. I don't intend to make it extremely detailed, but more-so as a way to explain some things about a couple of the most mysterious ponies ever mentioned.
Thanks to the following for finding errors:
Starlight Nova
EDIT: As I'm sure some of you have read the comics- namely the 'Fiendship Is Magic' series- you know that the story behind Sombra is different. I had written this beforehand and will just leave it up for others to read.
Chapters (1)
I liked the theory and the way you explained it in one chapter.
Just so you know, there is no "ed" at the end of Starswirl.
4289144
Thank you very much.
I felt like their weren't any good theories or stories about Starswirl, so I figured to try and start some.
4289472
Really?
Whew, thanks!
Dually noted and fixed.
Finally, someone else who believes Sombra is Starswirl!
4315616
Ah, so I'm not the first!
Sequel
4694629
Well, I wouldn't know where to start.
Usually when I write anything, I think about all possibilities, all plans, and all major events as well as small-time events that lead up to the major ones. In doing so, I'd have to answer several questions:
>) How would she go about doing it? (If I were to do it, I'd try learning about the whole time-travel/ time-distortion thing in the comics where Celestia and him talk
) And finally, what would finally help Sombra? (Does she protect him from some damaging spell or something and makes him care for her? 
)
What could happen for Twilight to help in changing Sombra? (Maybe another unexplained, never mentioned villain is attacking and Sombra may help as Discord does <I say that because I really hate when that happens
I would love the many stories that could be made, but I don't think I have the ability to come up with a clever enough story that'll keep readers entertained and enthralled until the very ending
. If you have an idea, I think you should give it a go and I'll be happy to give it a read! 
Fascinating...very intriguing. Now, it needs a sequel.