The Dark Exile was a time when the three Prime Evils were exiled to Sanctuary by the four Lesser Evils as a result of a civil war within the Burning Hells. The reason for the war was the Lesser Evils' concern that the Three had abandoned the war with the High Heavens and focused too much of their attention on humanity. The banishment led to the formation of the Horadrim on Sanctuary to hunt them down and imprison them inside the soulstones, which would eventually lead up to the events of Diablo I, II and III.
Following the end of the Sin War, a truce was brokered between the Angiris Council and the Prime Evils. The truce stipulated that neither the High Heavens nor the Burning Hells should intervene in the affairs of mortals and coerce them to choose a side in the Eternal Conflict, and furthermore that so long as the truce held the forces of Heaven and Hell wouldn't do battle with each other either. In effect, the truce led to a standstill in the Eternal Conflict.
 The Exile
The Three sought to use circumvent this truce in order to sway mankind to the side of Hell, but the Lesser Evils did not approve. They believed that demonkind was far superior to humanity, and that the Eternal Conflict was theirs to fight. In an effort to overthrow the Prime Evils and return the war to what it once was, the Lesser Evils, led by Azmodan and Belial, launched a rebellion agains the Three.
It is unknown whether the Three were prepared for this uprising, but though they used all their power and reportedly annihilated one third of Hell's forces in the process, the Three's own minions eventually turned on their masters together with the Lesser Evils. Weakened by this they were banished to Sanctuary. What happened next is uncertain. Some sources claim that that Belial and Azmodan, now without a common goal, turned on each other and continued the civil war, while others speculate that they are still in league with each other. Whichever it is, Azmodan has ruled Hell for the last 300 years since the Dark Exile took place.
 The Hunt for the Three
Banished to Sanctuary, the Three set about trying to corrupt mankind through subterfuge and by spreading chaos. They possessed a host of different humans, from high-ranking politicians to religious leaders in order to further their own goals, and their presence spread hatred, terror and destruction across the land for many years.
Decades later, Tyrael eventually came to notice that something was awry in Sanctuary, and quickly realized that the Three had been banished there. Fearful what would happen should the rest of the Angiris Council discover what had happened, he decided to try and amend the situation without alerting his peers. By doing this he hoped he could save humankind from eradication by the Heavenly Host, as would surely be the case should the rest of the Council discover the presence of the Three.
Thus he entrusted mankind with the task of capturing and binding the Three. For this purpose he formed the Horadrim, a legendary order of mages recruited from the destroyed remnants of the clans that remained after the Mage Clan Wars. Their members were very few, somewhere between seven and twelve, and all handpicked by Tyrael. To help them, he fashioned from the Worldstone three shards, one for each Prime Evil, that would become the soulstones. Zoltun Kulle, one of the Horadrim, was entrusted with carrying and using the stones to imprison the Three.
Given their task, the Horadrim set off towards Kehjistan where the Three were all located. The first to be captured was Mephisto, and the encounter ended in a bloody battle in one of the cities in the region. The stone with Mephisto inside was entrusted to the Zakarum religion, and they would later build the temple city of Travincal to guard Mephisto's soulstone.
Baal and Diablo eventually fled from Kehjistan to the western desert of Aranoch. Baal took refuge inside of Lut Gholein, but the Horadrim tracked him down and waited for him to leave the city. When he did, a cataclysmic battle erupted, and although Baal was defeated without civilian losses this time, his soulstone had been shattered in the process.
Though Baal was successfully captured within the largest shard remaining of the soulstone, the Horadrim realized it would not be enough to contain him. Desperate for a solution, it was Zoltun Kulle who theorized that a human heart might work as a surrogate for the shattered soulstone, if the human had a strong enough mind. Tal Rasha, their leader, volunteered for the task of imprisoning the demon lord within himself. He was chained and buried in a tomb far out in the deserts of Aranoch, and six other fake tombs were also build to hide the true one.
Diablo proved much harder to capture than his brothers, and eluded capture for nearly a decade. Eventually he was tracked down and captured in Khanduras, and a monastery was built atop a series of labyrinthine crypts in which he was buried. The monastery would later become the center of Tristram. As the final Brother was captured, the Horadrim were at a loss of what to do. Jered Cain, new leader of the Horadrim after Tal Rasha, decided that records should be kept of what they had done and set to formalize the order. Eventually however the Horadrim faded with no task left to undertake, and became little more than keepers of information before eventually some decades after capturing Diablo.
 Old Lore
 Diablo I Manual
"Seven is the number of the Powers of Hell, and Seven is the number of the Great Evils."
- Duriel, the Lord of Pain
- Andariel, the Maiden of Anguish
- Belial, the Lord of Lies
These are the true names of the lesser of the Great Evils. For ages uncounted each have ruled over their own domains within the Burning Hells, seeking absolute dominion over their infernal brethren. As the Lesser Four continuously vied for the control of those forces that dwelled within their realms, the Greater Three held absolute power over the whole of Hell. The Lesser Four used dark and evil measures in their quest for power, and herein begins the legend of the Dark Exile.
- Azmodan, the Lord of Sin
- Mephisto, the Lord of Hatred
- Baal, the Lord of Destruction
These are the Prime Evils of the Burning Hells that wielded their power as a dark, sovereign triumvirate. The Three Brothers ruled over the Lesser Four by brutal force and malicious cunning. Being the eldest and strongest of the Evils, the Three Brothers were responsible for countless victories against the armies of the Light. Although they never held sway over the High Heavens for long, the Three were justly feared by enemies and subject alike.
- Diablo, the Lord of Terror
With the ascension of Man and the subsequent standstill of the Great Conflict, the Three Brothers began to devote their energies to the perversion of mortal souls. The Three realized that Man was the key to victory in the war against Heaven, and thus altered their rigid agendum that they propagated since the Beginning. This change caused many of the Lesser Evils to question the authority of the Three, and so brought about a great rift between the Prime Evils and their servitors.
In their ignorance, the Lesser Evils began to believe that the Three were afraid to continue the war with Heaven. Frustrated by the cessation of the war, Azmodan and Belial saw the situation as their chance to overthrow the Prime Evils and take control of Hell for themselves. The two demon lords made a pact with their minor brethren, assuring them that the wretched plague of humanity would not deter the ultimate victory of the sons of Hell. Azmodan and Belial devised a plan to end the stalemate, achieve victory in the Sin War and ultimately ride the bloody crest of the Great Conflict straight into the very arms of Armageddon. Thus, a great revolution was set into motion as all of Hell went to war against the Three Brothers...
The Brothers fought with all the savagery of the Underworld, and to their credit, annihilated a third of Hell's treacherous legions. In the end, however, they were overcome by the Horned Death led by the traitors Azmodan and Belial. The Prime Evils, weakened and bodiless, were banished to the mortal realm where Azmodan hoped that they would remain trapped forever. Azmodan believed that with the Three set loose upon humanity, the Angels would be forced to turn their focus upon the mortal plane - thus leaving the Gates of Heaven abandoned and defenseless. Those few demons who still pledged allegiance to the Three Brothers fled the wrath of Azmodan and Belial, escaping to the realm of Man to seek out their lost Masters.
As the warfires died out upon the battlefields of Hell, Azmodan and Belial began to argue over which of them held the higher authority. The pact that they had made quickly fell to ashes as the two demon lords took up arms against each other. The legions of Hell that remained were polarized behind either warlord, launching themselves into a bloody civil war that has lasted to this day...
 Excerpt from the Dark Exile Tome
- "So it came to be that there was a great revolution within the Burning Hells known as The Dark Exile. The Lesser Evils overthrew the Three Prime Evils and banished their spirit forms to the mortal realm. The demons Belial (the Lord of Lies) and Azmodan (the Lord of Sin) fought to claim rulership of Hell during the absence of the Three Brothers. All of Hell polarized between the factions of Belial and Azmodan while the forces of the High Heavens continually battered upon the very Gates of Hell."
- Cain, Deckard., Book of Cain, page 81.
- Cain, Deckard., Book of Cain, page 86.
- Cain, Deckard., Book of Cain, page 87.
- Cain, Deckard., Book of Cain, page 90.
- Cain, Deckard., Book of Cain, page 87.
- Cain, Deckard., Book of Cain, page 88.
- Cain, Deckard., Book of Cain, page 90.
- Cain, Deckard., Book of Cain, page 91.
- Cain, Deckard., Book of Cain, page 92.
- Cain, Deckard., Book of Cain, page 92-93.
- Blizzard Entertainment, Diablo I Manual, page 63-64.