From the introduction: The year is 2199, the setting Earth. Almost a hundred years ago, a great calamity shook the world. A great monster, known as the Black Beast, appeared from out of nowhere and covered the world in a mysterious substance called Seithr. Harmless in small quantities but deadly in high concentrations, Seithr is addictive and can even cause one's body and mind to deteriorate. A great many perished to Seithr corruption before anyone knew what was happening, for Seithr cannot be seen or smelled.