Back in the time of the Four Tribulations.
Although the Mountain and Sea Realm had broken away from Hongyuan, it actually remained within the domain of the Primordial Path. It constantly sought to counter-attack Hongyuan and become the sole master of the Heaven and Earth Path.
Shanhai Pass.
This was the jagged front line where the two realms clashed. This war over the Dao between two civilizations was no small matter, destined to be a prolonged war of attrition lasting for hundreds of millions of years.
The two sides, attacker and defender.
They had been fighting since the Fourth Tribulation. They didn't even cease hostilities when the Fifth Tribulation erupted, and the sheer scale of resources and soldiers poured into the conflict was already unimaginable.
If the Pass stands, the realm survives; if the Pass falls, the realm is doomed.
