This is a request story from Chazkopa, and is an alternate ending to Canterlot Wedding. It's based on this, but when Twilight's about to kill Chrysalis, the changeling queen creates a portal and escapes, realizing that with Twilight so powerful in dark magic, she'd be unable to conquer Canterlot.

The Mane 5, Shining and Celestia apologize to Twi, but it's too late. She doesn't forgive them. Twilight angrily turns to them, and broadcasts a recording of what happened at the wedding rehearsal. The wedding guests were furious. Also, when she broadcasted it, not only Canterlot, but all of Equestria saw. And then...

Why don't you read the story now?