This story is a sequel to Dawn Of The Silver Sun
The Corrupted came from the west, that is all that's known. Before ponies understood what's going on, princess Celestia marched to stop them. She had to know something nopony else did, but it wasn't enough.
The last vestiges of ponies and changelings are now ruled by princess Luna, queen Chrysalis, and Twilight Sparkle, starving and losing hope in face of an enemy who grows stronger with every passing day.
Sex tag for: descriptions of bodyparts and base animal instincts. No clop.