Zoltun Kulle is one of the antagonists of the Diablo series. He is a former member of Horadrim clan.
He was one of the original, founding members of the Horadrim who took a precipitous turn towards evil when he discovered the black soul stone. A mage of immense power, at one time he was the most dangerous man in the world of Sanctuary. To confront this threat, the forces of Sanctuary teamed together and worked in vain to destroy Zolton Kulle, as he had already found out the secret to immortality. As a working solution to this, his head was severed and buried in the desert while his body was imprisoned in an alternate dimension.
He came from Ennead Clan, a clan of sorcerers and sorceresses, who were legendary for their work of mastering transmutation and other magic arts in common with that. Kulle spent his life pursuing this. Zoltun also had potential in alchemy and he was assigned to handle the 3 soul stones, which would be used to trap the 3 Prime Evils. During the time he spent with the soul stones, he got more interested in their power. Jerad Cain, Deckard Cain's ancestor, would see him experimenting and writing as of late with them.
As his knowledge grew the more attracted the other Horadim members became. However the quest for the Prime Evils made him insane, the other Horadim members told him they needed a very powerful object, which he gave them, The Cube. The other members hid it fearing the danger of being corrupt. After Diablo's defeat, Kulle took a dark path and left the Horadim.
Kulle returned to Kehjistan and it was there that he developed and created the black soul stone, a stone with enough power to capture the angels of the High Heavens and demons of The Burning Hells. The other Horadim members caught him during a summoning spell and after a long fight, they defeated him and took his head from his body and hid him within the Shadow Realm. At the time of the events of Diablo 3, Kulle agreed to aid the heroes on one condition that he be alive again if you know what I mean.
After the heroes find his blood and bring him back to life, he says to the heroes to quit their saving the world mission and conquer the world instead. The heroes defeat him and then take the soul stone.