The war between the Northern Reaches and the Southern Domain was started by the cultivators of the Northern Reaches after noticing the weakness of the Southern Domain after the Southern Domain Civil War. The Northern Reaches' main goal was the Immortal Destiny that was supposed to appear in the Southern Domain. The Northern Reaches received help from Dawn Immortal who wanted to absorb Meng Hao and make him feel the pain of losing everything.
After the Northern Reaches lost the war, all of their experts were killed in battle or captured and made into felon citizens and the path to Nascent Soul of their bloodlines was blocked by Meng Hao: no one with Northern Reaches blood in them could reach the Nascent Soul stage. Their peak dao seeking experts were trapped under Mount and their spiritual energy was used to restore the devastated and desolate lands of the Southern Domain for ten thousand years under a mountain. This mountain came to be known as Sin of the North.