In Canterlot Castle a purple unicorn by the name Twilight Sparkle lay crying after everypony had left her when she burst into the wedding rehearsel, and claimed Princess Mi Amore Cadenza, or Cadence, was evil.
Meanwhile the Changeling Queen Chrysalis, was looking at her after giving the others the slip. She had been pretending to be Cadence, while stealing the love of Shining Armor, Twilights brother, Captain of the Royal Guard, and Twilight's BBBFF, as she prepared an invasion.
She had been planning on sending Twilight to the caves within Canterlot, but had finally received the information on her she had requested of her changelings after Twilight almost ruined her plans, information that said not only was Twilight Shining Armor's little sister, but was also both the personal student of Celestia, and the bearer of the element of magic, as such she decided on a different method of getting rid of Twilight, one that makes it so that even if she managed to return, she would be unable to use magic.
With a smirk, and knowing she didn't have long, before the others returned, Chrysalis turned into Celestia and walked up to the sobbing mare, while secretly casting several privacy spells so the real Celestia would not sense what was about to happen.
"Twilight Sparkle." Chrysalis said.
"I'm sorry, I'm so sorry." Twilight sobbed.
"I'm sorry to my little pony," Chrysalis lied while acting like Celestia due to wanting Twilight to think she is Celestia, "but I have decided that with you declaring Princess Mi Amore Cadenza, a member of the royal family, as evil, while the city is under threat of attack, then you are in liege with our unknown enemy, and are hereby declared a traitor to Equestria."
"No! No! No!" Twilight cried. "I'm not a traitor, please believe me."
"As punishment," Chrysalis continued secretly enjoying Twilight's cries. "You are hereby banished from Equestria, but first to make sure you do not use the Element of Magic to return, I will be changing your species so you are no longer a unicorn, and therefore unable to use your magic."
Then while enjoying the cries of the hysteric Twilight, she used a changeling spell designed to trick ponies into thinking a non changeling was a changeling to draw away hunters who discovered them. A spell that truly changes a pony into another race, though for some reason what race they become cannot be controlled. And since it was for emergency escape, there was never a counterspell, or way to control the race created.
And so with a rush of flames from the spell, Twilight Sparkle the unicorn became Twilight Sparkle the pegasus. Then Chrysalis focused her magic again and cast the banishment spell she found in a spellbook, called spells of an Equestrian Princess, she found in Cadence's room from her princess studies, that not only banished Twilight, but made it so that unless Celestia, Luna, or Cadence cast the counterspell it is impossible for her to step foot in Equestria. And thanks to the privacy spells none of them would know.
And so Twilight disappeared into the vortex created to send her away, as Chrysalis became Cadence again. Not knowing that even with getting rid of Twilight, her invasion would still fail. Unfortunately for Twilight, without telling anyone what happened to her.