Ethan hadn’t seen Ashmedai’s power when he fought against the Progenitor of Pestilence, Erra.
He had fallen unconscious during that time, so he didn't see how the battle between two Progenitors ended.
However, Sebastian gave him a blow by blow, commentary of the fight, which made the young man wish that he had been awake to see it first hand.
Because of this, he knew that choosing recklessly wouldn’t benefit him.
So, before choosing one of the branches that was available to him, he asked Ashmedai to tell him the pros and cons of each branch of magic, so that he would be able to decide which one would suit his style the most.
The Demonic Progenitor was more than happy to explain his powers in great detail.
Mimir, who was serving as a witness to the eminent pact between Ethan and the Demonic Progenitor was surprised because he thought that Ashmedai would throw some half truths into his explanation.