The Birth of the Beast is an unlockable video, or cutscene, in God of War. A player can unlock it by beating the game on any difficulty. It reveals Kratos' past as a child as he and his younger brother, Deimos, are being ready to be trained to fight in the Spartan Army. Kratos had been well fit to become a soldier, but his brother had been too weak and died in the mountains. His brother had died and fell to Hades, growing of age and seeking revenge upon the brother who abandoned him. The events of this video was retconned later in God of War: Ghost of Sparta, where the brothers were attacked by Athena and Ares , who eventually kidnapped Deimos to take him to the Domain of Death. As in the original video, Deimos grew up in this realm and came to hate the brother who abandoned him.
Video
http://www.youtube.com/watch?v=psKDIKW1XuY