The reason why Ethan was willing to become a Core Disciple was because of the monopoly that the sects held.
Whether it was a new secret realm or a cultivation resource, it would always be the top three sects that claimed ownership first.
Even though Ethan had enough strength and trump cards to fight over them, it would be far too risky once the organisations backing these sects decided to act.
Moreover, Ethan didn't want to always fight… especially against the top sects. His goal was not to destroy them outright, but to slowly infiltrate and take over the sects—one step at a time—until he became the true ruler of the Emerald Green Plains.
For now, his immediate focus was simple: use the Greenfire Sect's intelligence network regarding the Night Demons and complete his first aggressive mission—[Save 100 Night Demons].
