Zhu Youcan was dazed.
Was this still not enough?
"Brother Wang, I've already transferred all the slaves from the planets around here. Dark Energy-level slaves are rare, to begin with," Zhu Youcan explained. "In such numbers, they're only available on the battlefield. As a lieutenant colonel, you should know that it is easy to kill enemies, but the requirements for enslavement are too high, even more so when enslaving Dark Energy-level beings."
Of course, Wang Ye knew.
One needed to have a great advantage on the battlefield, and one's mastery of the Enslavement Spell be very high. In addition, one's soul must not be weaker than the enslaved, and it would be best if it was an entire level higher.
For example, to enslave a Dark Energy level 9 demon, one basically needed a Super Star-level ability user who was proficient in the Enslavement Spell.
On the battlefield, those with such ability were at least sub-lieutenants.