kings

There are like a 15-20 kings in a standard game.

Of those 6 hold pieces of the demon lord's soul in a doll they own. Those dolls act as the "royal guard" of the king.

The Solfrey cannot be controlled by a necromancer but the guard with a soul can, this is a way the empress keeps control over the king.

Once all 6 of those kings are identified and killed, the demon king is reawakened.