"I know the Black Soulstone is the key, and it shall be mine! Soon, my armies shall pour forth from the shattered mountain, ravaging this world and all hope of resistance. My minions will find the Stone, wherever you choose to hide it. Then, at long last, Azmodan shall reign as the Prime Evil."— Azmodan to Diablo (through Leah)(src)
The Black Soulstone was an artifact of incredible power created by the renegade Horadrim known as Zoltun Kulle. Unlike the other Soulstones, the Black Soulstone was intended to hold the souls of many beings, angels and demons alike, and in particular was capable of trapping all seven of the Great Evils. In this it was successful, but backfired when Diablo used the artifact to make himself the singular Prime Evil, and further backfired when the archangel Malthael seized the stone and altered it in order to use it as a weapon to destroy all humanity.
"I spent many hours with them as we hunted the three. I studied them, intrigued by their power. In the long years after our victory, I felt an emptiness within me, a bitterness that consumed my days and nights. With a soulstone of my own, Heaven and Hell could be brought low and humanity could rise. Unchallenged. Unrestricted."— Zoltun Kulle(src)
When the Horadrim were tasked to capture the Prime Evils and imprison them in the three original soulstones, Zoltun Kulle was charged with bearing the artifacts. The search was long and arduous, but eventually, all three of the Primes were imprisoned. Kulle, however, was left a broken man, and the Horadrim did not hear of him until reports reached them that he had built an archive below the sands of Kehjistan. Jered Cain believed that Kulle had become obsessed with soulstones, and so was attempting to create one of his own—the Black Soulstone, capable of harnessing the essence of many angels and demons. Containing these essences, filled with rage, hope, fear, and valor, Kulle would use the soulstone as a substitute for his own black and empty heart. The Black Soulstone was a "profane simulacrum" of the original soulstones. Its construction remains a mystery, though Deckard Cain uncovered stories of Kulle forging the Black Soulstone from the bones of fallen magi. Another tale claimed Kulle extracted his own blood and crystallized it into the Soulstone. 
By his own account, Kulle created the soulstone to ensure that humanity could rise above the shackles of Heaven and Hell. After all, the soulstones containing the three Prime Evils still remained in Sanctuary itself, and if not for Tyrael, the forces of Heaven would have wiped out humanity long ago. He believed that he could help mankind unlock its nephalem birthright and enter a golden age. Regardless of intent, the Horadrim feared that as a result of his experiments, he might effectively become immortal. They killed Zoltun Kulle, separating his head from his body and draining his blood.
Return of the StoneEdit
Centuries after his death, Zoltun Kulle was resurrected by the Nephalem, seeking to defeat the last two lords of Hell, Belial and Azmodan, in exchange for guiding the adventurers to the Black Soulstone. Thanks to the witch Adria — who had learnt of the stone's existence at the same time of Diablo's awakening in Tristram and had marked the spiritual essences of the other Evils into the stone — the souls of the five already defeated Great Evils (the Lessers Andariel and Duriel and the Primes Diablo, Mephisto and Baal) had been drawn into the Black Soulstone the moment it was finished.
Kulle, upon realizing what has happened, attempted to keep the soulstone from his former allies. However, he was defeated, and the stone used by Leah to imprison the sixth of the Great Evils, Belial. The now unstable stone, writhing with the essences of six Lords of Hell, was subsequently used to imprison the final Evil, Azmodan, who had failed in his siege of Bastion's Keep.
Soon, however, Adria showed her true colors and allegiance: Leah had been bred as a vessel for Diablo, and the essences within the Stone were channeled into her, giving rise to a new Diablo, combining all seven essences into a singular Prime Evil.
Despite this, the Nephalem were able to defeat Diablo. As his body fell from Heaven and disintegrated, the Black Soulstone was seen falling through the clouds below.
The Corruption of HeavenEdit
"I cannot destroy the stone, nor allow its evil to remain within the Heavens. And so it must be hidden. Even from the angels."— Tyrael(src)
The Black Soulstone remained in Heaven, still containing the Seven Evils. Since it was a man-made object, Tyrael did not know if he could destroy it, and, given its different nature from the previous soulstones, was unsure if that was a wise course of action. While the Luminarei guarded the artifact, the Angiris Council debated over its fate. Imperius wanting to destroy it, Auriel wanting to seal it, Itherael paralyzed by indecision. Tyrael, the only one able to see what the stone was doing to the angels, insisted that it be hidden on Sanctuary, but was rebuffed. All the while, the soulstone's corruption spread, even tainting the Crystal Arch.
The Reaper of SoulsEdit
"Nephalem...the Black Soulstone journeys through the portal to Sanctuary. Even now, it spreads death across your world. And so ends the Eternal Conflict."— Malthael(src)
With the aid of Horadrim, the stone was to be placed in the Tomb of Rakkis. While the Black Soulstone was being put into place, Malthael revealed himself, having taken on the Aspect of Death. The Horadrim were killed (except Lorath Nahr) and Tyrael was incapacitated, leaving Malthael free to take the Black Soulstone for himself. Tyrael was able to retrieve a sliver of the artifact, which fell off when Malthael took it.
Malthael fundamentally altered the Black Soulstone in order to use it as a massive soul trap that could drain demonic essence from all creatures in a single plane of existence. By the time the Nephalem made it to the heart of the Pandemonium Fortress where the Worldstone once stood, Malthael had released the soulstone into Sanctuary with the intent of using it to tear the demonic essence from every human in the world, killing them by tearing their bodies and souls apart. The two did battle, and in a last-ditch bid to defeat his foe, Malthael summoned the Black Soulstone back into the fortress and shattered it, embedding its shards into his body and taking on the power of the Prime Evil. After his defeat, Malthael's body disintegrated, Diablo was released, still containing the essences of the other Evils.
- ↑ Book of Cain
- ↑ Book of Tyrael
- ↑ 3.0 3.1 Diablo III, Act II
- ↑ Diablo III, Act III
- ↑ Diablo III, Act IV
- ↑ Diablo III Ending Cinematic
- ↑ 7.0 7.1 7.2 2013-08-21, Diablo III: Reaper of Souls Opening Cinematic. YouTube, accessed on 2013-08-22
- ↑ 2013-09-12, Deckard Cain was the last living Horadrim. Blizzard Entertainment, accessed on 2013-09-15
- ↑ 2013-08-13, Deckard Cain was the last living Horadrim. Blizzard Entertainment, accessed on 2013-09-15
- ↑ Book of Tyrael
- ↑ Storm of Light
- ↑ Diablo III, Act V
- ↑ 2014-05-19, More Info From The Anniversary Dev Stream. Diablo Fans.com, accessed on 2014-05-20