The natural progress of the universe was disrupted by the EVENT, splitting the future into two distinct possibilities, and creating two sentient beings — referred to in the story as the Voice or the Spirit of the Light or Dark Prophecy.
Each Voice/Spirit works through its representative, the (current) Child of Light or the (current) Child of Dark to attempt to have their future achieved, and repair the damage caused during the EVENT.
There appear to be fundamental and philosophical differences between the two prophecies. The Light Prophecy tended to work through a variety of agents, even having multiple concurrently-alive people who were the Child of Light e.g., Belgarath, Poledra, Belgarion, Eriond. The Light Prophecy also tended to seek progress and strove for worldwide peace. The Dark Prophecy tended to have singular individuals as the Child of Dark and namely Torak and then Zandramas, who then passed it to Prince Geran before the Final Event. The Dark Prophecy also strove for domination / dominion including dominion over (like "smothering of") the Light.