Dark Celestial Magic | Tyrrapedia: Encyclopedia of Tyrra

Dark Celestial Magic is somewhat of a mystery. Just as Earth Magic has its opposite (Necromancy), some believe Celestial Magic does as well. Just as Necromancy ignores protectives and armor, so does this "dark" celestial magic... Its other known characteristic is that each time it is cast, it causes a measure of backlash of damage of like kind to the caster, sometimes a small percent of the initial output of the spell, sometimes as much as the entire spell.

The closest, most logical explanation of the source of Dark Celestial Magic is that it is drawn from the darkness between the stars. This would suggest that it is not "celestial necromancy", as some refer to it, but rather a different focus of celestial energy (unlike Necromancy which calls upon the Void of Corruption drawn through the Plane of Chaos). The other theory, according to Hollym, is that it is simply an intensifying of celestial energy to such a degree that it affects both the caster and target.