Summary: Once upon a time, when Twilight Sparkle was still in school and became Princess Celestia's assistant, she got a boyfriend! His name was Fassbender, a male unicorn. Fassbender and Twilight got into a lovely relationship. Became very close.
Even though, Twilight was mostly kept busy as Princess Celestia's assistant and started seeing less of Fassbender until she realized that their relationship wasn't working out. When she tells Fassbender that they need to break up, he leaves Equestria all together in disgust and claims to never come back.
Many years later, when Fassbender finds out that Twilight moved onto some other pony like Flash Sentry, he returns to win her back. Fassbender is not really a ordinary male unicorn. For the whole time, he's actually a vicious, intense demon who's always wanted to become the new ruler of Equestria and eliminate Princess Celestia somehow. When he found out that Twilight became Princess Celestia's apprentice, he disguised himself to blend in with the pony world.
The reason why Princess Celestia never suspected that Fassbender was going to pull off a terrible tragedy to her kingdom is, of course she only knows him by his real, true appearance and his nickname, "the Scoundrel."