Starswirl the Bearded finds a book about dark magic and is tempted to use it. After using it for sometime, Celestia summons Starswirl to her to ask what hes been up to. After he reveals what he's been doing, Starswirl ventures to the Crystal Empire to drain the Crystal Heart of its power. He then becomes King Sombra after things go wrong
Wow, great Sombra theory! Other than "a few minor mistakes" it was really good but a little short. Why don't you try expanding it a little.
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I'll try when I get around to it.
And thanks.
-slow, honorary clap- Great job, in my opinion.