The Dark Imperium - From The Ashes

Return of the Jedi AU
In the throne room of the second Death Star, Anakin Skywalker turns back to the Light Side of the Force, and kills Emperor Palpatine to save his son, Luke Skywalker. But what no one realized is that all of Anakin's anger, hatred, and other dark emotions had attached themselves to the Dark Side, and had culminated into a new entity.
Vader.
Vader has returned from the dead, and with the help of Grand Admiral Thrawn, and a host of loyal Imperials, he is forming the Dark Imperium, a new faction to replace the Empire, and destroy the Rebel Alliance. And with Anakin dead, there is nothing holding the Sith back from sending all of his enemies into oblivion.