Cain's reaction mirrored that of the others in the hall—his eyes sharpened, his expression growing more solemn. A Demon King wasn't a race but a title given to an extremely powerful entity, typically from an enemy army.
If someone as powerful as Lady Alume referred to an entity as a Demon King, it implied that this being's strength could rival ancient life forms like the Star Bound Sovereign. Worse still, if this Demon King's castle lay inside the Primeval World, it might be connected to the Great War—a time in the Crimson World when even Void Eclipse-level entities perished on a daily basis.
