Question about Death Armor Spell

[Anamnesis]

What makes Death Armor a Necromantic Spell?
 
 Is there some unseen effect that it does to the subject of the spell that could be clarified, that would make it necromantic rather than abjuration or evocation? The current description in lore doesn't expand on what makes it necromantic and curious minds want to know.
 
 Caster Level(s): Wizard / Sorcerer 2
Innate Level: 2
School: Necromancy
Descriptor(s): Magical Armor
Component(s): Verbal, Somatic, Material
Range: Personal
Area of Effect / Target: Caster
Duration: 1 Hour
Additional Counter Spells:
Save: Will negates
Spell Resistance: Yes
Material Component: A Black Onyx
 
 A magical aura surrounds the caster -- injuring creatures that touch it. Any creature striking the caster takes 1d4 points of magical damage +1 point per caster level (maximum +10). This armor also grants a +2 natural armor bonus.

[Skywatcher]

I thought the reason was that the magical aura was sort of a force field of negative energy drawn from the realm of the dead so someone was injured when they encountered it.  I am not sure what the official explanation would be though.

[Dorganath]

Necromancy is magic that affects life and life force.  Death Armor saps that from a creature who impacts it, thereby sapping a bit of life, which is registered as damage. This is its primary mode of operation.  NWN doesn't have a "death" damage type, so we use "magic" in this case.  It's not really "negative" energy as such.

The AC increase is a secondary effect.

I'm pretty sure that description is stock NWN, and sadly, there's no stock D&D spell by that name which I can reference.  By convention, however, when something describes a "death" effect, whether instant death or a gradual damage to one's life force, that falls under Necromancy.