Half-demon, half-human, Louie Marcillen had lived peacefully with his mother in the Kingdom of Menfil until the Majin disease struck him. Physical changes, caused by this disease, led to persecution of Louie and rape and death of his mother. Powerful demon god Graza, the father of Louie, took care of him in his dungeon and tried to put out the Louie's fire of hatred towards humans. This was ended futile, as the dungeon was raided by human Heroes, slaughtering everyone on their way including his father. Louie who was lucky enough to survive, made a vow to revenge humans for his parent's death. As the demon's blood is burning in his veins, there are still memories of his human mother, her love and warmth. Struggling with these contradictory feelings, Louie begins his journey towards the Menfil's capital.

Born as a "half-demon" with the blood of demon god, Louie Marcillen lost his parents due to human's violence and human's Heroes. Four years have passed since he finished his revenge... The Kingdom of Menfil, now led by King Louie, still despised as a "demon", was in blockade by adjoining countries starting with the Kingdom of Karssha. By the efforts of Louie's aides blockade was lifted, but he was accused of murdering of Master Teneira right in the peace conference. Being struggling between Light and Darkness in his heart, Louie is forced to make a decision as a king. The future depends upon it. The future of his beloved Queen Irina, and future of warriors and princesses who choose the side and fight for Louie or against... The Phantom War... The battle, later so called, will lead to the unrevealed truth of Ekria Teshuos, the Princess General of the Kingdom of Karssha.

After fierce battles between gods, two different worlds joined to become a new one. Celica was born there and to protect innocents, he battles against monsters as a paladin of Barouhart, god of storms. With a clear sense of justice and great skill with the blade, some call him a Hero. However after he meets a wandering seeker of knowledge and is tasked with purifying an ancient evil artifact, he begins to question his beliefs as he is drawn and more into events involving ancient prophecies, ancient gods and demon gods. This is the prequel to the Ikusa Megami Series and covers the events that led Celica to be known as a God Slayer. An append disc, sold together with the Ikusa Megami Zero: Complete Guidebook (ISBN: 9784903491868), adds extra special items and a supplementary realm, "Mugen Kairō" (夢幻回廊, or Corridor of Dreams). It also bumps up the game version to 2.0.