Ouch, I can imagine the heartbreak and fear Princess Celestia is feeling when she realizes what's become of her sister. It had to be quite a shock, and I'm sure the riding crops of harmony were the last thing on her mind until she at least had a chance to talk to Luna/NMM. The stormy night outside the old castle or temple wall. (Architecture came a long way in the next millennium.) It was a horrible evening in every way.