Sunset's uncovered and stumbled on quite a few secrets in her years of tyranny but she's put all of that behind her...or has she? After all, secrets are very useful tools.
Sunset Shimmer accompanies Pinkie in stalking Sonata... Er--wait... Sunset Shimmer tries to protect Pinkie from Sonata Wait...No... Sunset Shimmer watches several men die of heart failure. There we go. That's the one.
The longer Equestrian magic runs wild in a world that wasn't designed to house it, the bigger the danger. Sunset must find a way to contain it. She just has to.