Everypony knows that there is nothing as terrifying as Princess Luna's/Nightmare Moon's wrath.... except her sister Celestia, who becomes a demon that could make a hydra sit in time-out willingly every twenty eight days. During this time every month, all of Canterlot is evacuated except The Royal Sisters and specially trained Guard members. But what happens when one unfortunate stallion doesn't get the memo?

Collaboration with Shardz.