Necromancers were few in numbers to say the least, and it wasn't without proper reasoning backing the statistics up.
These people were beings that devoted themselves to mastery over life and death. To learn such skills, they too had to tread upon the thin line that separated life and death. Meddling with the taboo.
Thus, typical necromancers would also originate from an undead descent.
Although necromancy was not too fondly looked upon because of its usage and costs, to master the art itself was actually quite a harsh feat to do. However, upon mastery, you would be able to make use of the corpses you come across that fulfills certain qualifications.
Those who knew necromancy and those who mastered necromancy were different. Because this was a B-ranked dungeon, Aria suspected that the final battle will be against a Lich. Spellcasters that were undead themselves. They would be classified into beings who 'knew' necromancy depending on their level of skill.