Zargoth Plane.

"It's good for Cain that he came here. He gets to swap out a Legendary Oath from the demon lord with a Divine Oath from me. It is his luck. I can't wait to bring that luck to her other subordinates. They deserve a better option." Aeternus said to himself.

Seeing Cain's talent made him realize how rich the demon lord is in talented subordinates. Her Oath of the lord might be a lesser rank than his but she has had thousands of years to accumulate the best talents in the plane. Forcing various geniuses in the domain of high-rank demons to be his subordinates won't make up the difference. But forcing her subordinates to become his might just do the trick.