Many millennia ago, the demonic army known as the Burning Legion formed under the dark titan Sargeras. The Legion set out to destroy all life in the universe. They crushed planets, annihilating all races and species in their wake. However, the Naaru have spoken of a glimmer of hope, an Army of Light that will destroy the Legion.
No being knows of when that day will come, but until then, a group of heroes will rise up and defend Azeroth from the Legion and all other evils that would threaten life. This group is known as the Knights of the Light.