Necromancy was a form of magic frowned upon because of its harmful effects on the universe. The world follows a cycle of Life, Death, and rebirth, a cycle necromancy magic breaks.
By holding onto the soul and forcing it into servitude, its reincarnation cycle had been broken. This was the only form of magic the Õnis frowned upon and fought against openly.
Necromancy Magic was closer to Death Magic than any form of dark magic. To safely extract and bind the souls of the dead, one needs to tap into the true death magic, exposing himself to harm.
The exposure to death magic corrupts the caster's soul over time, tainting it with the death magic they wields. The longer they hold onto the soul of the dead, they begin to lose their humanity, experiencing emotional detachment, paranoia, and nightmares.
