Great Dark War

The First War of Ars Magus (第一次魔導大戦, Dai-ichiji Madō Taisen, lit. First War of Magic), alternatively known as the Dark War (暗黒大戦, Ankoku Taisen, lit. Great Dark War), was an event spanning a decade and took place almost a century before Calamity Trigger. A monstrosity known as the Black Beast appeared in the year 2100 and it's only goal was the complete annihilation of mankind. The Black Beast spread a substance known as seithr all over the world, as that was the only way the Black Beast could operate. The Black Beast's power was overwhelming, and killed off more than half of the world's population. Mankind tried to do everything they could to eliminate the seithr, thinking it would be easier and safer than fighting the beast directly, but even that plan failed.

One Year Preparation
For exactly one year, the Black Beast had disappeared. In this short time frame, humanity was able to recuperate and made and gathered many weapons for when the Beast would appear again. Eventually, the Six Heroes appeared and the situation took a turn. Konoe A. Mercury of the Six Heroes passed her knowledge of magic, the mystic force of the universe, to mankind. Unfortunately, magic couldn't be used in its basest form by humans, as they were unable to manipulate and control it. Humans did a great deal of research into the subject and eventually developed something known as Ars Magus.

End of the Dark War
After one year of inactivity, the Black Beast resurfaced. Unfortunately, Ars Magus alone wasn't enough to defeat the Beast, so mankind created something more powerful, called Ars Armagus, which could be used to directly combat the Beast. After the humans figured out how to use Ars Magus and Armagus, they fought alongside the Six Heroes; the Black Beast was finally defeated in the year 2110 AD.