To see somepony die is painfull. To see somepony die slowly in front of your eyes unable to do anything but watch their life fade away, that is tormenting.
Yet it is this torment Rainbow Dash is experiencing when Applejack is left grievously wounded; knowing her own failure doomed her marefiend. Now Rainbow Dash is willing to do anything to undo what was done, even if it the price turns out to be her very life.

To turn away, and walk a different path is not something everypony does. So how could the farmpony Big Macintosh end up doing so? And what does his father and an organisation called "The defenders" have to do with this?

After a particularly taxing tour of duty outside Equestria, Twilight is more than happy to be returning to Ponyville for a well-deserved rest. But six months away is a long time, and the reunion with one particular pony proves capable of sending her life in a direction she wouldn't dare to dream of, both for the better and for the worse.