The war of Heaven and Hell started when the Demon Lord had tried to escape the under world. This angered the other gods. The then-demon lord pulled Raven out from his storage in hell to fight. Raven then after some fighting, overthew the demon lord, stopped the war by surrendering, and then began to shift his focus to Aef.
This particular event was very important to Raven's development, as this is how he came to be King of Hell.