All living things, even plants, have energy signatures, except for a handful of humans known as Voids, who are born with no spiritual energy what so ever. The rarest of all humans, Voids have no magical powers or gifts of any kind, but neither are they affected in any way by immortal abilities, magical powers, or mortal gifts.
Only one human in five hundred thousand is born a Void. And of those, few survive to adulthood. Immortals, and Weavers to a lesser extent, consider Voids a threat, an aberration, an abomination to be destroyed. Because Voids have no spiritual energy of their own, they must feed on the energy of others in order to survive, absorbing it through their skin like some amphibians absorb oxygen. Simply by touching them, a Void could potentially drain all the power from an immortal, Weaver, or even gifted human. Rendering them mundane mortals if the Void stops in time. And killing them if he or she doesn't.
Despite their rarity, though, Voids are not generally hard to find. The gifted, Weavers, and immortals can immediately sense it when they're in the presence of one. Just like they can usually sense power emanating from others, like energy radiating from a tiny sun, they can also sense the black holes that are Voids. Even mundane humans and animals can sense Voids to some extent, even if it's just a general feeling of unease when in the Void's company. Perhaps it's an extant predator sensor.
Voids are born only to pure human parents, but once they reach adulthood, they stopping aging like any other immortal. Voids have their powers since birth, but only to a limited extent. They still need the energy of others, but they lack the strength to drain anyone to death. They tend to be sickly children, pale and anemic because their ability to siphon energy from others is barely strong enough to sustain themselves. But they're immune to any normal human illness and rarely die of natural causes.
Most of them are unaware of what they are until their powers start to grow during puberty. They tend to be thought of as weak and tiring children. Few parents would believe their child to be an ultra-rare Void. Most mundane mortals think Voids are a myth.
Although Weavers fear them and immortals hunt them, there are some who believe Voids are the gods', or perhaps nature's, way of providing balance to the world. The priests and priestesses belonging to the temple of Kuroshu in particular believe this. The servants of Kuroshu, a goddess of justice, scour the planet for Voids, hoping to gather them into the protection of the temples ahead of any immortal's blade.
Reapers, the sentinels belonging to the gods of Death, guide the souls of the dead to the netherworld. But it's Kuroshu who determines where each soul ends up. In statues and other art, she is portrayed with four arms, the head of a horned owl, and feathered wings billowing behind her like a cape. She is bone white, except for her ink black eyes, beak, and talons. On the right side of her body, she holds a sword in one hand, pointing accusingly down at anyone kneeling before her. In her other right hand, an anatomically correct heart is held aloft, with rivulets of blood rolling down her forearm. On the left side of her body, a feather is raised aloft in one hand, held above the set of scales held outstretched in the other hand.
It is believed the heart is the seat of one's soul. And the weight of the deceased's heart determines whether the soul spends eternity in the elysian fields of paradise, the hallowed halls of heroes, the cold, barren wastelands of purgatory, the frozen depths of hell, or somewhere in between. All souls can choose to be reincarnated, however, and return to the mortal realm. But the heavier one's heart, the lowlier a creature they are in their next life.
Despite her fierce appearance and the fear she inspires among mortals, the priests of Kuroshu believe their patron deity is not a vengeful god. Like the scales she holds, Kuroshu represents balance. And the priests of Kuroshu believe it is their role, and their duty, to uphold balance. Especially among the immortals, who tend to believe their powers render them above mortal laws.
And so these warrior priests and priestesses have taken it upon themselves to police the immortals, with the help of their Void charges. In most places, temples are considered sacrosanct and operate nearly autonomously. (And those few places that have tried to bring the temples under the control of local ruling bodies have usually met with revolution. Temples wield immense power, and often immense wealth, and those who forget are quickly schooled.) And so when a temple decides to enact a vigilante policy, there's not much the local authorities can, or will, do to stop them.
In some places, the priests of Kuroshu are the sole sources of law enforcement. But all temples, even those dedicated to the same god, are generally operated independently, and they can vary wildly between locations. In Blackmire, the temple of Kuroshu only steps in when the High Priestess believes the Guard hasn't made sufficient effort. Guards aren't particularly pleased when Kuroshu priests step on their toes. And they're especially displeased when the Voids leave the temple grounds. But the Metropolitan Guard has a general policy of noninterference regarding temples and their priests.
These priests and priestesses refer to themselves simply servants, but mundane mortals have romanticized them as the Knights of Kuroshu. Some consider all of Kuroshu's priests and priestesses "Knights," but some only use that title when referring to the Voids in particular.
Regardless of what they're called, the servants of Kuroshu strike a fearsome sight when they're on a mission. Like their goddess, they are adorned in white robes (white is associated with death.) Their natural skin tones are hidden beneath the ashes of their fallen brothers and sisters. Their faces are covered by helmets made to look like the heads of horned owls, and their armor is white steel shaped like bones. Their finger nails are grown long like talons, then sharpened and hardened with black lacquer.
Whether mundane mortal or Void, all of Kuroshu's priests are trained in the martial arts. The ageless Voids grow ever more proficient with time and experience. They are slightly stronger and faster than the average human, especially after a "meal." But their only real defense is their ability to render immortals impotent.