At the beginning:
The Demon King's Subordinate: He's just a useless Demon King.
Everyone in the Holy See: He's just a useless devil.
Later:
The Demon King's subordinate: As expected of the Demon King.
Everyone in the Holy See: He is indeed the Dark Lord.
This is the story of a devil who thinks too much and conquers the world.
Purpose: The devil is engaged in infrastructure construction.