Nagol one of the last crusaders of Light leads his order to defeat the Demon king, with heavy casualties. Nagols order is reduced to three survivors but just as the Demon king is slain the United Kingdoms of Order turn on the Crusaders destroying the proud order of Light.
Witnessing his own mother slain then his best friend before he himself was brought down by the soldiers he had fought side by side with. His Rage and Sorrow caused the Goddess of light great suffering, and so her brother The God of Death took him to be forged into the King of the undead.