Divide And Conquer.

The stronger his subordinates are, the better for him. But according to Helios's information about the gods of the plane, the available divinity from killing all the gods won't be enough for all of the demon dukes to reach the half-divine rank. Rarely is any resource enough. That's why there is a need for competition and the survival of the fittest. So only the best will get divinity. 

The mortal plane is the demon duke's playground for now. They will return to their appropriate battlefield when they are done here. He will test them thoroughly there and those that he finds wanting will pay with their lives to boost their betters.

Elsewhere All Around The Plane. Immediately Aeternus Entered The Plane.

The plant spirits guarding mana entities transforming into demigods all received the same order from the tree father. They are to kill their charges and move on.