Necromancers participated in the Great War of Magi but they joined hands with the creatures of the dark. In the end, they were defeated.
The sorceresses and warlocks that participated in that war, saw how the necromancers utilized vile methods of using soul arts and summoning the dead to fight with them
They also witnessed the depraved act of life extension. Because most of the necromancers that joined the war during that time support Schoolmaster Yin's research.
Life extension plays a vital role in why the new mage community decided to obliterate the necromancers once and for all.
Little did they now that the actual proprietor of that wicked magic spell is still set on a loose and was not punished in the least.
The young Yin Shun tried his best to hide his identity as an apprentice necromancer which does not seem to be hard for him.