The Great Calamity... A war? A plague? Perhaps something more mysterious? All we know is that the calamity ended with The Five, the godly beings that created the built our world from the ashes of the old, creating the new Races and blessing us with the Magic that permeates our world. It's also said that the One Chained in the Dark rose at this time, but was thwarted by the Five - chained in a demiplane of complete darkness. There it's influence over the world was lessened, though not completely destroyed.