"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 Leah
The Black Soulstone is an artifact of incredible power, capable of trapping all the Great Evils. In this it was succesful, but backfired when Diablo used the artifact to make himself the singular Prime Evil.
"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."
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 a 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. In contrast, 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 defeated and imprisoned Kulle, seperating his body as due to his experiments, he had effectively become immortal.
Centuries after his imprisonment, Kulle was released by a group of adventurers seeking to defeat the last two lords of Hell, Belial and Azmodan. Kulle was resurrected in exchange for guiding them to the Black Soulstone. Kulle obliged, but found that the souls of the other Great Evils had already been drawn into the stone. This was thanks to the witch Adria, who had learnt of the stone's existance at the same time of Diablo's awakening in Tristram, and had marked the spiritual essences of the other Evils into the stone, where, once in it, they could be destroyed permenantly. Kulle, who turned on the heroes, was defeated again, and the stone used by Leah to imprison Belial. The now unstable stone, writhing with the essences of six lords of Hell, was next used to imprison Azmodan, who had failed to obtain it in his siege of Bastion's Keep. Here, 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. A singular Prime Evil.
Despite this, the nephalem heroes were able to defeat Diablo. As his body fell from Heaven and disintergrated, the Black Soulstone was seen falling through the clouds below, possibly disintergrating as well