Bael Turath is a historical empire from the past of the Nentir Vale setting of Dungeons & Dragons. Founded by humans, its ruling caste were transformed into the ancestors of all tieflings through diabolism and an unholy pact with Asmodeus.
Ironically, despite being so integral to the history of the tieflings, Bael Turath was left fairly ambiguous to most casual players, a result of the PHBs' focus on crunch over fluff. The first titbits were hinted at in the "Wizards Presents" duology, with further fleshing out appearing in articles in Dragon Magazine issues #381, #383 and #387. The greatest details were revealed in the mini-sourcebook "Player's Handbook Races: Tieflings".
The origins of Bael Turath are lost to the ravages of time and war. All that is known is that it was an early civilization of humans who banded together and lifted themselves to the heights of glory, conquering many lesser kingdoms in the process. Eventually, they reached the peak of their possible power... and, like all empires that have risen far, a fall was inevitable.
Descent into Darkness
The usual array of troubles bedeviled Bael Turath as it wavered; the pressures of ruling defeated territories strained it, noble houses selfishly plotted and warred for greater power or the destruction of perceived enemies, civil wars, attempted secessions, assassination plots, famines, plagues, etcetera. It was enough to make the nobility of the empire struggle with restless dreams, up to and including the empire. They were desperate for a way to preserve their empire, and cried out in their dreams for a way to save it.
And someone heard them. The emperor of Bael Turath, and the most avaricious, domineering and paranoid of his nobles, were contacted in their dreams, fed visions to what they believed was the key to the survival of Bael Turath: a pact with the Nine Hells. Without hesitation, Bael Turath's elite prepared for a month-long ritual, the Bloodfire Moon, that would open the portal to Hell and allow them to cement this grand-scale infernal pact. To their credit, many of the noble houses protested this, but they were silenced by the damned majority; the lucky ones died fighting their former comrades, the unlucky were dragged to the altars and sacrificed as proof of Bael Turath's commitment.
Many and varied were the fiendish pacts sworn through the Bloodfire Moon, with dusty tomes committing to memory ominous titles such as the Iron Crown of Madness, the Scarlet Claw of Hunger, Night's Loving Void, the Million Pains of Eternal Torment, the Heart of Fire and Iron, and the Cage of the Unnamed Master. The specifics are lost to all but the devils they were sworn with, but their legacy remains obvious.
The nobility of Bael Turath were transformed into something new, humanoids with the powers of devils. Backed by swiftly-growing schools of infernal magecraft and armies of devilish warriors, Bael Turath was revitalized and surged forth into the world again. Decades past as the empire grew and grew, its diabolic aspects becoming normalized and celebrated - after all, had not Asmodeus been true to his word? Were they not mightier now than ever before?
But, in the end, it was but a delay of the inevitable.
The doom of Bael Turath came when the empire came into conflict with another great power: Arkhosia, a rising young star of dragonborn ruled over by a rare coalition of both Metallic Dragons and Chromatic Dragons. The two powers took an instant offense to the other, skirmishes and small clashes ultimately growing into what became known as the War of Ruin. This was a conflict of incredible proportions, where diabolic magics and armies of slave-soldiers clashed against draconic might. It was brutal and merciless, fought without pity or quarter.
And, ultimately, it was the doom of both. In the end, they couldn't sustain the sheer scale of butchery, leaving both factions to collapse into ruin. The once-proud tieflings were forced to scatter, save for those rare domains where they were welcomed as protectors against the chaos ravaging the former empire. In the end, Bael Turath took its place in the realm of ruins; neither the first nor the last of the great empires to fall.
Much of Bael Turath's lore has been lost since the War of Ruin. Only fragments, many jealously guarded by tiefling families, still linger in the world today.
Whilst all tieflings ultimately descend from one noble house or another, only a handful remain relatively known today.
Achazriel, The Destroyer's House, may not even be a House; these tieflings claim descent from Achazriel, the brilliant and ruthless tiefling general who finally ended the ten-year seige of the Arkhosian citadel of Razortear by leading a charge of ten thousand slave-soldiers into it through an underground passageway.
Khalir, The House of Blood, was considered lurid and depraved even in the height of Bael Turath's powers. Led entirely by an internal caste of tiefling vampires, it is rumored that obscene necromantic rituals allowed these blood-drinking fiend-touched to actually procreate, proliferating a tainted bloodline of tiefling dhampirs and necromancers that is said to continue to this day.
Dreygu, The Lover's House, earned its title through the story of two star-crossed tiefling lovers; Ivania of Dreygu and her beloved, Vorno of Kahnebor. Imagine if Romeo & Juliet had been savvy and ruthless enough to survive the fighting of their houses, outwit the assassination attempts, and come to dominate their two houses, forcing a merger through marriage and by murdering all of their kin who dared to object.
Kahnebor, The House of Feasting, was a family of hedonism and depravity that even the tieflings of Khalir looked askance at, host to rumors so dark that even today, few tieflings will ever voluntarily confess to tracing their lineage to Khanebore.
Zannifer, The Red House, is of little repute and import, save for the story of a family curse that forces those of its bloodline to either wear red or murder a criminal once per month, lest they begin constantly oozing blood.
Zolfura, House of Ice and Fire, was a powerful house of arcanists with a strong natural affinity for the destructive potential of elementalism.
Whilst Bael Turath is dead, some of the teachings of its more powerful institutes remain, preserving fighting techniques and combative skills honed under diabolic tutors.
The Turathi Hell-kite Paragon Path represents the most simple of those legacies, preserving a martial arts style designed specifically to take advantage of tiefling biology, from the use of the tail to sweep foes from their feet or pull allies to safety when you dodge, to crushing skulls with a horn-buffered headbutt, to tapping into the infernal energies boiling within all tieflings to coat a weapon in hellish flames.
In the empire's heyday, the Crimson Legion were some of its most elite - and feared - troops; an order of multi-classed and hybrid-classes Paladin-Warlocks sworn to the Lord of All Devils. The practice of combining infernal warlockery with divine might remains; the loyalty to Asmodeus is gone. Viewing their former patron as having betrayed them, these tieflings instead serve as an unlikely medium between gods who wish to oppose Asmodeus and the myriad devils who wish to betray him.
This antipathy towards their former racial patron drives many of the few tieflings who seek divine power to pursue the path of the Hell's Keeper; a vengeful spirit seeking to become a divine jailer, in order to transform Baator into a place of suffering, torment and imprisonment for the devils that claim to rule it, whether or not they belong to the Crimson Legion.
Whilst the Bleak Academy of old may be gone, fragments of this devil-tutored arcane school's teachings remain, ensuring that tieflings still have an inherent advantage in the fields of magical fire and fear.
Whilst few tieflings ever hear the call of the Primal Spirits, those who do - mostly barbarians and shamans - often become Redeemers of the Damned, exploiting loopholes in infernal contracts granted by their connection to the Spirits to draw the spirits of their ancestors from the Nine Hells and give them a chance to find redemption from the torment they suffer there.
Although most think of the tieflings as a fairly monolithic race, the reality is that many houses formed unique pacts with various archdevils or other patron fiends, or engaged in unusual practices. The subspecies of tiefling may go unnoticed in the crowd, but they are there.
The Blood-Crowned Courtiers have actual devil's blood in their veins, allowing them to assume the form of powerful fiendish versrions of themselves when roused to battle.
The Broken Mirrors bear a pact to Levistus, causing misfortune to follow them like a loyal dog.
The Blightseers are heirs to a pact with Baalzebul, allowing them to see the decay and degeneration of all things - and to force what will be to become what is.