Zaos wanted to make Milliendra practice controlling her power as much as possible. However, he knew that once Drannor heard about him, he would organize the largest army that the world had seen. So, with that in mind, Zaos had to alert other mercenary leaders. While the group didn't fight directly against orders and grew a lot, there were three other groups that were even larger than Zaos'. They should be able to cause a lot of trouble to Drannor, but that wasn't Zaos' goal.
"Millie, first Let's check if your power is the same with low physical contact," Zaos said. "Touch my hand with a single finger."
Milliendra nodded, and even though she hesitated, she touched his hands, and she could feel Zaos energy being drained. She tried to stop it, but it was impossible… In any case, Zaos noticed that he didn't lose all that much mana. So that confirmed the theory the mana drained relied on the level of body contact.