Blood-Curse of The Lower Realms

Those suffering the Blood-Curse of the Lower Realms are most commonly referred to as Tieflings. They are one of three varieties of Half-Breed Sentients, along with Aasimar (those suffering the Blood-Curse of the Higher Realms) and Skinwalkers (those suffering the Blood-Curse of the Bloody Moon). As Half-Breed Sentients, they are subject to the Rule of Eight, so Tieflings always beget Tieflings for eight generations. Tieflings can originate from any mortal race, but most commonly are the spawn of wicked humans.
Tieflings have a wide variety of appearances, but are always obviously fiendish in origin. They are often physically twisted, and more often than not have horns on their head as a defining feature (though the shape of these horns are just as varied as Tieflings themselves). A common misconception is that all Tieflings resemble humanoid pit fiends- with red skin, a very human appearance otherwise, a devil's tail, horns, and hooves. The reality is that while these particular Tieflings are fairly rare among their own kind, they are often the only ones able to fully integrate into society. Many Tieflings are simply lynched when their curse becomes apparent, or they are otherwise hidden away. A common escape from such a fate is for grotesque Tieflings to enlist with their local military- Tieflings among all Half-Breed races are disproportionately represented among city guards and similar low-level military forces.

Causes

Like the other spontaneous Half-Breeds, Tieflings can come about in one of four ways. The most common is that the Tiefling is descended from a fiend of some variety, and these Tieflings will always take an appearance similar to their true origin. Such Tieflings are not Half-Fiends, that is to say that neither of their parents are fiends, rather a grandparent or more commonly distant ancestor lay with such a creature. 
The second most common origin of the Blood-Curse is for the parents of the Tiefling to be granted a divine boon from an evil-aligned deity or high-ranking fiend. As a reward for services rendered, a powerful entity such as a god or pit fiend may curse an unborn child conceived of devoted followers or soul-sellers. Similarly, the third origin is that some same entity might inflict a Tiefling upon a family that had attracted the fiend's wrath. 
The final and least common origin of Tieflings is that an adult with no fiendish affliation might be cursed as punishment by a powerful deity such as Fortissimus or KuQala. This event is extremely rare, so much so that there are only four verifiable cases of such since the Imperial Census began under the reign of Zhao III.

Symptoms

Tieflings most often are born looking like a normal member of their host race, and the curse emerges slowly during adolesence. This is an incredibly painful process both physically and emotionally. Many are driven out by their communities at this time. The entire transformation takes anywhere from one to five years, and often renders the victims traumatized. They also begin to feel nearly overwhelming rage, lust, greed, or such similar emotions, and more often than not are driven to act on these violent impulses. 
Some Tieflings are born already manifested fully, and these have a much easier time than their grown kin. All Tieflings are born understanding either the tongues of The Abyss or the Lowest Hells, though most do not realize it (it has been described as hearing the language as though it were their mother tongue, as through the use of a spell of linguistics). All Tieflings born of Tieflings as a rule are born corrupted, always manifesting the same mutations as their parents (the mother in the case of two Tieflings breeding). 
While those born already corrupted have an easier physical existence than those who fully manifest the curse later in life, but have a closer connection to the Lower Planes of existence. They often hear the voices of fiends urging them to act as a proxy for the fiends' desires in the Material Plane, and some become mad by growing up with voices in their head before they are capable of thinking.

Treatment

There is no way to reverse the curse itself save from direct divine intervention from a good-aligned deity, which has only been recorded as happening exactly once in the recorded histories of the various churches. However, when a Tiefling commits their life and soul to such a deity or one of neutral alignment, they are said to experience a calming of their fiendish impulses, and those that exist in physical pain that walk this path oftentimes have this pain eased. However, the stigma against Tieflings is such that many temples will refuse to take them in.

Type

Supernatural

Origin

Divine

Rarity

Extremely Rare