I've been searching for over a week so far with no luck, and I really hope someone has at least some knowledge of this story.

What I remember is, Luna finds out that her actions as Nightmare Moon, the castle collapsing and so on, accidentally killed somepony, and she goes back in time in a desperate attempt to change things. She tries to talk to Celestia, who doesn't believe her, tries to find her past self in the cave she had hidden in, but she's not there, and when she tries to directly intervene, she herself is almost crushed by falling rubble, and the pony who she had tried to save pushes her out of the way, dying anyway in her place. Luna then reveals when she gets back to the present, that that pony who died was an ancestor of Blueblood's.