Boredom.

Cultivation solved all the problems,

The weak Union members started to become stronger, which eventually strengthened the Union more than ever.

Not only that, but it even started increasing the lifetime.

The more one cultivated, the stronger he got and the longer he lived.

This was simply amazing and the Union got another boost.

In a matter of a few decades, the Union conquered over 90% of Yrniel.

The Union Leaders were happy as well, everything was proceeding smoothly, there were no problems at all, they didn't have to do anything, the Union members were able to take care of everything, even if it was a Ruler Class Beasts.

As for the Lord Class Beasts, well, all of them were hunted down.

The Union Leaders saw them as threats, therefore, they went out of their way to go, find, and hunt them down.

Just like how they did things in the beginning.

Therefore, the Union faced no problems and didn't bother the Leaders much.