Note: This story was conceived of long before the second EQG movie was created, so it's a bit outdated compared to later information that emerged. It just took me a long time to finally write it.
It should have been a simple affair. Wait for the portal to open again. Apologize for the past to Celestia. Move on as friends. Sunset Shimmer has been waiting for the opportunity to make amends for her past misdeeds, having learned of the magic of friendship from her new friends. She wants to deal with her regrets and right her wrongs.
She never expected that things could go so horribly wrong in two days. It should have been simple, but a discovery of a familiar dark artifact renders things more complicated than she could have ever foreseen. All she wanted was the power to protect her friends from their enemies, but now she is the enemy. Can Twilight Sparkle and her friends save Sunset Shimmer from herself a second time?