Necromancers—or, as they are sometimes called, undead summoners—are one of the professions tied to dark spiritual energy. People wouldn't view this profession so negatively if all the necromancers didn't try to attack the safe zone.
More importantly, humans have limited capability to pursue this path. Only a few become completely obsessed with dark spiritual energy and receive assistance from creatures of the dark realm can they become necromancers.
Fortunately, necromancers have a weakness to light. Even powerful ones can summon undead that operate during the daytime, but they still suffer from the light element.
It's not always the light element tied to sunlight, though. It could be a holy light, which only people with the priest profession can wield.
The good news is that you can still kill necromancers and their undead without holy light. Just be extra careful at night, when the undead grow stronger.
