As the empire return so did the King of Fear; King Sombra. As he returned, the Empire has fell and Twilight Sparkle is believed to be dead. A year later, Twilight returns. But she isn't the same pony as she was before.

King Sombra has return. The Crystal Empire has fallen and Twilight Sparkle is believed to be dead. Years later 'She' returns, but not the same.

"The wrong mentor can corrupt, even the purest apprentice" Princess Celestia

There's a Monster In All of Us. And they want out no matter the cost. - Princess Luna, Princess of The Moon

Some say King Sombra is a unicorn that only wants to enslave the crystal ponies. Celestia said His heart was black as night.