Everypony must live with their choices. In Twilight's case, she must live with it forever, literally. Becoming Princess of the Afterlife, of Tartarus, however, was just half for atonement. She had another agenda. She wished to seek out the departed souls of her friends, to beg their forgiveness, just one last time. 10 years have passed and her search is still fruitless.
Now on the 10th anniversary of her friends' passing the true nature of their sacrifice is revealed. She is given the chance to make the choice once again. This time, will she choose Equestria or her dear friends?