The story of Diablo is based on the premise of a war between Heaven and Hell

The story of Diablo 2 items is based on the premise of a war between Heaven and Hell. The town of Tristram has come under attack by demons, and the player must save the town and, in effect, the world, by ridding it of the Lord of Terror. Also as quoted in a Warcraft 2 preview of Diablo, the hero is also there to avenge his family who died at the hand of Diablo. As the player delves into the underworld, some of the history behind the war between Heaven and Hell, as well as knowledge about Diablo 2 items himself, are revealed through large tomes that are found throughout the levels.

Diablo 2 items is the Lord of Terror and one of the Three Prime Evils of Hell, the most powerful lords of demonkind. Long before the events of the game, he was captured by a secretive order of mortal magi known as the Horadrim. The Horadrim imprisoned each of the Prime Evils in a Soulstone; Diablo's red stone was buried in caverns deep beneath the town of Tristram, and as the generations passed, was forgotten. Though his imprisonment was meant to be eternal, the power of the Soulstone weakened over centuries, eventually allowing Diablo to use limited power from within the stone. He telepathically turned an inhabitant of Tristram, the Archbishop Lazarus, into his pawn. In order for Diablo 2 items to actually leave the Soulstone, Diablo needed to possess a host. Through his minion Lazarus, he initially tried to gain control of King Leoric, the local ruler, but Diablo, in his weakened state, was unable to overpower Leoric. Abandoning the idea, he caused Lazarus to kidnap King Leoric's son, Prince Albrecht. He inspired such terror in the child that the boundaries between the realms were broken and parts of Hell appeared in the mortal world, taking root in the labyrinth beneath Tristram. Diablo 2 items then chose to bide his time and wait for the opportune moment to strike.