History.
In the dark days, when hordes beyond the veil assaulted Nhera, venturing out of Valurn meant encountering lands of nothing but blood-soaked ash and stone.
There was no sustenance to be found. Nothing, save blood.
Blood, whether of Nheran creatures or any living being, contains aether, the life force essential to creation.
Thus, the first Vampires were born. Starved of aether from bleeding in defense against the dark forces, some elves consumed enough infernal, aether-laden blood to be transformed. No longer able to tolerate the warmth of Asra as before, they were barred from returning to Valurn once the veil lifted and light shone on the Darklands for the first time.
Even other races, with less sensitivity to aether – humans and dwarves, for example – could become vampires. This requires drinking the blood of an infernal lord, receiving their blessing willingly, or being bitten by such transformed creatures.
Regardless of race or origin, Rendyres must consume aether through blood or succumb to wretched unlife and wither away.
Those bitten have little time to seek a place of celestial power, such as those dedicated to Asrani light worship. These might include Human churches or Eldar starwells and moonwells.
It happens often that many elves, particularly Drâihari, are not aware of their Rendyr status- as often blood drinking and many other behaviours of Vampyres are reflected commonly in their culture.
Result.
Vampirism, known among scholars of the unseen as "Aetheronemia," is not a curse of demonic possession but a profound metaphysical affliction. A living being in the Nheros Realm Matrix constantly and passively absorbs life-giving Empyrean Aether from Asra, much as a plant absorbs sunlight. The Rendyr, or vampyre, is a being whose connection to this source has been violently severed or pathologically damaged.
Unable to process the raw, overwhelming torrent of life energy from Asra, direct exposure is agonizing to them. It is not the heat of a star that burns them, but the unbearable pressure of pure vitality trying to enter a sealed system, causing their Telluric form to overload and decay. To stave off their slow dissolution into dust, they must acquire aether that has already been processed and contained within a living vessel. Thus, the eternal Thirst is born. By consuming blood, they steal a finite, second-hand dose of life force. This act is purely parasitic; it is a temporary patch on an unending spiritual deficit, forcing them into a desperate cycle of hunting and feeding to keep their own decaying nature at bay.
Hyuron Vampirism.
While the curse of Aetheronemia can theoretically afflict any mortal race, its successful manifestation is far from universal. The ancient and numerous Drâihari Rendyr stand in stark contrast to the profound rarity of their Hyuron counterparts. Scholars of the occult and hunters of the night alike attribute this discrepancy to the fundamental, warring natures of the Hyuron soul.
The Eldar physiology, for all its tragedy in the fall, is ancient and possesses a degree of ordered resilience. When an Eldar is afflicted, their soul, while severed from Asra, can often find a new, terrible equilibrium. It re-centers itself around the Cold Thirst, creating a creature of patient, eternal predation that, while monstrous, is relatively stable in its new existence.
The Hyuron soul offers no such peace. To inflict the vampiric curse upon a being with latent Beast-Saint ancestry is to ignite a civil war within the blood. The cold, entropic emptiness of the vampiric state clashes violently with the Hot Fury of the Hyuron's primal reversion. It is an unending battle between the predator that stalks in silence and the beast that howls in rage.
For most Hyuron, this discordant union is unsustainable. The transformation frequently results in one of two catastrophic failures:
Physical Dissolution: The body simply tears itself apart, unable to contain the warring supernatural forces.
Monstrous Devolution: The subject devolves into a creature of pure, mindless savagery, possessing the thirst of a vampire but the indiscriminate, feral rage of a Varghast—a being often called a "Blood-Ghoul" by hunters, feared for its utter lack of reason.
The few Hyuron who survive the transformation become paragons of torment. They are wildly unpredictable, consumed by warring hungers, and prone to spectacular fits of both icy cruelty and bestial frenzy. This inherent instability means they often burn out their own existence in a brief, bloody storm, making them a volatile and short-lived phenomenon. For this reason, a Hyuron bearing the vampiric curse is seen not as a true immortal, but as a walking, ticking harbinger of their own violent demise.