Ages ago, there was a conflict between the Olympian and Japanese Gods. Both Zeus and Amatsu-Mikaboshi asked the Blacksmith to forge great weapons for them that would help them win the war.
The Blacksmith forged two swords: Kusanagi or Grasscutter for Amatsu-Mikaboshi and Godkiller for Zeus. As years went by Godkiller was lost. Kraken somehow came into possession of the sword and brought it to Japan when he went to recruit Gorgon. Kraken claimed that the sword is older than memory and had seen war on five different continents, killed kings, emperors, and once made a god bleed. He handed the sword over to Gorgon.
As the owner of the sword, Gorgon did not always use it, saving it for special occasions. When he went to fight Phobos, he took Godkiller with him and did battle with the Son of War who was armed with Grasscutter, Godkiller's ancient counterpart. During the intensity of the battle, Godkiller and Grasscutter clashed one last time and both of them snapped.