A Fierce Spirit is a type of incomplete Spiritual Body.
Skeletons have ranks, such as Withered Bone, White Bone, Gray Bone, Silver, Gold, Mourning, Black Crystal, etc. Zombies are classified into Tough Skin, Copper Skin, Iron Skin, among others. Naturally, Spiritual Bodies are no exception, though their classification is rather special.
Among them, the lowest rank—Wraith—is practically equivalent to mosquitoes in the Undead World. They're ubiquitous; wherever the breath of death accumulates, a swarm of Wraiths can sprout. They screech endlessly, buzzing around you and robbing you of peace.
Why is the breath of death on the Deadman's Fruit considered the purest? Because those deathly auras won't give rise to Wraiths.
For an Undead wishing to slumber peacefully, the best solution is to plant Cemetery Grass all around their grave. If not, even a Golden Skeleton could lose sleep because of the nuisance of Wraiths.
