Their teacher was one of the famous Legendary Masters in Mifata Federation, Master Mill.
This master was known as the "Sage of Life and Death," and he had carried out profound research on the two types of spells involving life and death. However, he had always taken his stand with the side of "living." Although he dabbled in spells pertaining to dead souls, he had never taken the initiative to turn a living being into a dead soul.
The fact that Tu Ya'an became a necromancer had already made Master Mill quite displeased, to the extent where Tu Ya'an found it embarrassing to continue staying in the Mill City. He found a place that was significantly far outside the city and built his own magic tower.