"Fire" is the first of the Elemental Domains, under Spirituality. Thus, the newly born consciousness of the undead can also persist within the Soulfire.
Although the undead have "spirit" in their name, most of them are not spiritual bodies, and are essentially still existences in the material realm.
The only difference is that, while the bodies of normal lives are 'alive,' theirs are 'dead.' But from another perspective, perhaps in the eyes of the wraiths, it is the bodies of the living on earth that are 'dead,' and theirs that are 'alive.'
As time passed, the number of wraiths gradually increased. Not only the corpses of lifeforms, but also places dense with Negative Energy brought forth life under the Nether Moon's illumination.
A few enlightened beings bathed in the moonlight, regarding the Nether Moon as the source of life. The wraiths call it the Primitive Moon, because in their blurry memories, there was a moon on earth too, but it came later than the one in the Underworld.
