The Kōma War (Japanese: 降魔戦争 Romaji: kōma sensō, translated in English as The War of the Monster's Fall by Software Sculptors) was one of the greatest conflicts in the Slayers world. It took place roughly 1,000 years before the birth of Lina Inverse. Partly because of its historical importance, KanzakaDex uses the war as an epoch to measure time from.
The war had four phases. First, Hellmaster staged a conflict between the humans in the peninsula which was later surrounded by the mazoku barrier. The war was a long and deadly feud, and left the human nations seriously weakened and their best fallen. Still, the local ryūzoku, elves and dwarves did not interfere yet, so in order to keep them occupied, Fibrizo started the second phase, and the peninsula was overrun by lesser demons. While the newly formed alliance of humans and other races fought against the demon onslaught, the more powerful mazoku were free to act as they wish and began the third phase. The five subordinates of Ruby Eye gathered in the Kataart Mountains, now mostly abandoned by ryūzoku troops, and began to weaken the Aqualord's forces by systematically destroying her temples and killing her priests. Most of the knowledge of the human practice of holy magic might have been lost in the process. When the ryūzoku managed to respond, they were too late, as the last phase already began and a piece of Shabranigdu was reborn. Sometimes, only this last phase is referred to as the Kōma War by certain sources.
Although even he could not know for certain, Fibrizo theorized that a piece of Ruby Eye was sealed away in the famous and powerful sorcerer Lei Magnus, so he tried to use the horrors of the the war to weaken the seal. His assumptions were correct, and Ruby Eye was awakened. Knowing that their plan of destroying Ragradia was surely discovered by now and they will be soon confronted with the ryūzoku and the shinzoku of the entire world, the mazoku put a barrier over the peninsula and managed keep the enemy's reinforcements out of the battle. Because of this, however, four of the five lords were unable to participate in the war anymore, so priests and generals were created in order to represent them. The ryūzoku were able to eliminate Fibrizo's priest and presumably Dynast Grausherra's general and priest as well, but Xelloss gave them a stinging blow by destroying thousands of dragons with one swipe of his finger. Meanwhile, Shabranigdu and the fifth lord, Chaos Dragon Garv, appeared in the Kataart Mountains and challenged Aqualord Ragradia. During a fierce battle, Aqualord was defeated, but she still managed to wound Garv, force him into a human body and seal the piece of Shabranigdu into a block of magical ice far to the north.
In the end, the war had no definite victor. With the loss of Ragradia, the shinzoku and the ryūzoku were greatly weakened, but the mazoku suffered losses as well: Fibrizo and Dynast lost both of their servants (how the former's general was destroyed is not known even by the dragons), Garv turned against the others by exploiting the advantages of his curse, and Lei Magnus-Shabranigdu was unable to leave the place he was imprisoned in. (Because of that he later gained the title of The Demon King of the North.) Also, the Aqualord did not disappear completely: She left her knowledge in an entity known as the Clair Bible, guarded by Auntie Aqua, the remaining thoughts of Ragradia herself.
The effects of the war could still be felt even hundreds of years later. The rise of Shabranigdu caused new, strange and frightening creatures such as trolls to appear, who cause trouble for humans ever since. Their losses caused inner conflicts to erupt among the ryūzoku, which led to the massacre of the ancient dragons. As the barrier remained in place even after the war, holy magic became uncastable with the Aqualord's death. However, the conflict was the cause of beneficial developments as well: In the next thousand years, magic flourished among humans, who became expert practitioners of white, black and shamanistic spells, while on the other side of the barrier, magic never really managed to evolve and people instead turned to technology.