It is said that once, centuries ago, a great pestilence came to this continent. 
Millions fell dead in its wake, and Wizard and Muggle alike found themselves 
powerless to combat it. In desperation, the greatest wizarding families sent a 
delegation to the centaurs, in the hope of discovering a cure. The centaurs 
agreed to help, for the disease had claimed many of their own as well.

The centaurs are skilled in divining cause and fate, and they told what they 
had discovered: that the Dementors were the source, for they were Death 
embodied, and the plague grew as their numbers grew. The humans had not 
considered this possibility, and they held counsel for many hours. Finally they 
devised a plan, by which the two races might accomplish what neither could do 
alone.

The wizards and witches combined their strength, and produced a formless 
Patronus of unequalled power. The centaur chieftain, sacrificing his life to 
wield that light, leapt into the sky, driving the Dementors before him. The few 
that remained would soon be hunted down and corralled by your Ministry, on the 
island that would become known as Azkaban. The plague had been defeated.

But the spell that brought victory exacted a heavy toll, and those who had cast 
it lay dying under the trees. Others of the noble houses arrived then, and 
seeing the still bodies of their kin, presumed that the centaurs had killed 
them. They came against us with anger in their hearts, but they were few, and 
to a man they fell. So the secret of the Dementors was again lost to wizardkind.

The spirit of the centaur chief, gazing down upon the field of battle, was 
filled with sadness at the death that had occurred there, and vowed that the 
banished Dementors would never cast their shadow on the world again. He still 
watches over us as the stars of Sagittarius, the centaur-archer. They shine 
upon the earth, reminding us that Death is not unstoppable, that it can be 
turned back.

And before the centaur, at the spot where he aims his bow, there is a tear in 
the world, a place of death, surrounded by a halo of light. It is all that 
remains of the scourge that once swept over this land, all that remains of the 
vanquished Dementors. It is guarded well by the Patronus and the archer, the 
spirit of wizard and centaur, who once worked together against Death.