What if the Massive Chalice that keeps the Immortal King alive is the cause of the demons attacks? Perhaps it's corrupted or maybe it's simply a demonic artifact to begin with. The longer the King survives due to the power of the Chalice, the stronger the plague on the land. Given that, the IK would know that the only way to protect his people in the long run would be to build up the bloodlines to the point where they have the power to gather the materials and ritual tomes required to cleanse the Chalice. When the Chalice is cleansed/purified, the demons would lose the ability to enter the world (or cease to be created by the Chalice?) and the game is "won".