The story I am about to tell you is classified as Top Secret.You have studied dementors. You may have wondered why, since no one you've heard of has ever met one. The true danger of dementors is not in your textbooks.They can breed. They cannot die.To breed they need a source of food: human emotion. From the oldest records that mention them until roughly 2000 of the Gregorian calendar, they were contained behind wards on a small island with limited food sources. Their numbers were constant.Then a Dark Lord freed them to serve in his terrorism campaign. His name was Voldemort. You've read about him, in your Profiling Dark Wizards course. He was no more evil than the hundreds of other Dark Lords that had come before him, but he was more foolish. He believed he could use the dementors; make them serve his purposes and his purposes only. Folly is far more dangerous than evil.He fell. You've read about that too. And the dementors remained free.Had the surviving wizards made gathering up the dementors a priority, they could have done it. But their institutions were in shambles and they were facing every possible humanitarian crisis. Apparently none of their leaders were trained in mathematics. The number of dementors born in any year is proportional to the number that are free, whereas the number that are captured is proportional to the number of wizards capturing them. Once the number that are free becomes too high...But they didn't think that way. It was two years before they made any serious effort to control the threat. By then, a few who were trained in muggle ways of thinkingdone the math. They proposed an all-out desperate effort. Every man, woman and child capable of wielding a wand was to be conscripted into the dementor-hunt. As best we can reconstruct, this plan had a good chance of working. Instead, the leadership opted for a compromise, making a “reasonable” effort in line with “common sense”.To this day, we disapprove of compromise, and of “reasonable common sense”.A year later, it was too late. Three years after that, they admitted it was too late. The shattered remnant of wizardry went to the muggles and told them the truth.The next three decades were a slow fighting retreat. Scientists searched frantically for a technology capable of harming dementors. Entire cities were used as bait and then consigned to nuclear oblivion. It didn't help. Nor could they increase the number of wizards. Magic may run in families, but it doesn't come from genes.The only thing that worked was leaving empty spaces. Dementors were slow to cross those. Again and again, quarantine lines were drawn, often at terrible cost. Those on the wrong side were abandoned to their fate. And again and again the dementors eventually crossed, and spread again.The end came in a city called Perth: with a vast desert on one side and a vaster ocean on the other. This was by design. Fifteen years before the end, while most of the planet was still habitable, they began one last project there. One last hope. Humanity, with maybe a hundred thousand total and a few hundred wizards, huddled around that project inside a circle of patroni. Every day the circle shrank.Then one day humanity's final hope was ready. The Great Ark launched from the very center of the town. When it reached an altitude of six hundred meters, the wizards on the perimeter apparated aboard. Many of them made it. The technicians running the launch tower had no such option.And that was the end of Earth-that-was.To this day, we do not know where the first dementors came from: whether by evolution or act of wizardry or natural magic. Nor do we know whether they can cross space, or if so how fast. We have not seen them here, but we dare not let down our guard.Because we do not have the industrial capacity to flee again.This is why the Alliance Ministry of Magic has dementor-watch as its highest priority. This is the real reason the Alliance could not permit the outer worlds to secede.Most of you are going on tours of duty soon. Many of you are headed for Reaver space. Many of you are afraid. I do not blame you.I only ask that you remember Earth-that-was. You are humanity'sline of defense. Frightening as Reavers are, the thought of not going should frighten you more.A/N Harry Potter and Firefly are the properties of their respective owners -- not me.