Fight Or Die.

A demon Duke is not a new term in the abyss. There can only be one demon lord but there can be others who have reached that power by relying on other options apart from the energy wwell.The subordinates of demon kings are demon dukes. They evolve to that rank from being demon nobles.

The demon noble rank is the last rank a demon can evolve to on its own. They usually need the assistance of the energy well of an abyssal plane or the power of a demon king to reach the next level.  They become demon dukes if they are assisted by a demon king instead of demon lords that use the energy well.

Aeternus can perform this assistance because he has something that demons kings should have, an infinite supply of energy at the half-divine rank. Even if he has chaos energy at the divine rank, Infernox will still be unable to evolve to the half-divine rank. Upgrading to half-divine and above requires the use of divinity. It is a must.