Twilight Sparkle becomes a changeling and soon finds her metamorphosis doesn't only affect her.

Twilight Sparkle finds a book by Clover the Clever on Princess Celestia that changes her forever

Nightmare Moon's return leaves her weakened, and is drawn to Twilight and the darkness in her heart

Alternate Harmony: Twilight discovers that the Elements and now themselves must obey Nightmare Moon!

Twilight is pulled into a future where her friends have become powerful enemies