Ever wonder where this random unlineaged alicorn had come from? Well Discord had destroyed all the others so she couldn't possibly be from Celestia or Luna.
She was actually a changeling who had been inadvertently summoned by Twilight in her early years and was stuck in this alicorn form, needing a powerful, binding love from this world to regain her powers, and, she thought, why not take over this world with her changeling powers?
She found a way, and that way was through her own summoner's brother, who was ripe in age.
Now, older, Twilight has discovered her folly and "Cadence's" plan, and could only think of one other thing powerful enough to stop a changeling with a source of love: another changeling.
Twilight forbiddenly summoned two more changelings to even the odds, to save her brother from this arranged, malevolent marriage....