The Legend of the Ogre Battle details events said to have taken place in the distant past. Mankind was engaged in war with the forces of evil--the denizens of the Underworld--with the victor ruling the world. Mankind drew power from their gods, while their opponents drew power from the Abyss.
When the Gods saw that the humans were on the verge of defeat, they sent the armies of Heaven, the Three Knights and the Twelve Disciples to the earth, to save humanity from destruction. The Gods also forged the Grail to help the humans defeat the Ogres.
Because humanity refused to stop fighting amongst themselves even after the Ogres were defeated, the gods vowed to never again aid the humans. The thirteen Disciples remained behind to spread knowledge among the people, while the Knights returned to the heavens.
As the forces of evil fled, they foretold that a second Ogre Battle would occur. Knowing this, one of the knights, Fenril the Ice, left the sacred sword Brynhildr with the humans so they could call upon the Gods should the need arise.
Indeed, one of the Disciples became obsessed with the power of the Abyss, and created an artifact--the Black Diamond--that could reopen the Chaos Gate. The remaining Disciples created the twelve Zodiac Stones to counteract the power of the Diamond.
Using the Black Diamond, the sage Rashidi and the summoned dark god Diablo came close to starting another Ogre Battle during the events of Ogre Battle: March of the Black Queen. They were defeated by the Xenobian Rebellion, and Diablo was sealed back into the underworld.