First Mana War
Conflict that happened around 1000 WB. Using a form of magitek. a powerful weapon was created, the Mana Fortress. Mana itself rebelled against this atrocity, creating the Mana Beast to set against it. Both the Mana Fortress and the Mana Beast ravaged the world; civilization in the Mana Dimension was all but wiped out. Eventually, however, the Mana Knight wielding the Mana Sword put a stop to the conflict. The First Mana War was far more destructive to the dimension than the Second Mana War that happened around 500 WB.
As a result of this highly destructive conflict, the dimension entered a dark age lasting several centuries (see Mana Dark Ages), and much of the technology and learning of the previous centuries was lost (including, thankfully, the knowledge used in the creation of the Mana Fortress.)