Necromancy is an ancient school of magic.  Since the dawn of time, magic users have been using magical techniques to control and interact with dead spirits.  Some ancient cultures held a great respect for such magic users, but in Medieval Europe, the attitude was different.  In both Christianity and the popular versions of paganism at the time, Necromancy in any form was looked down upon.  Opinions ranged from seeing it as disrespectful to the deceased to seeing it as actively unholy, but seeing it as a negative was universal.  That all changed with the death of Ivar the Boneless.