The town of Tristram is under attack by demons, and the player must save the town. As the player delves into the underworld, he discovers more about the demon Diablo, through large tomes that are found throughout the levels.
Detailed Diablo History
Diablo, an incredibly powerful demon, is the Lord of Terror and one of the Three Prime Evils of Hell, who had been imprisoned in a Soulstone and buried in caverns deep beneath the town of Tristram. Though his imprisonment was meant to be eternal, the power of the Soulstone weakened over centuries, eventually allowing Diablo to awaken from his forced slumber. He telepathically turns a nearby human, the Archbishop Lazarus, into his loyal puppet. In order for Diablo to actually leave the Soulstone, Diablo needs the stone to be shattered. He possesses Lazarus, and has him break the stone. Diablo then tries to gain control of King Leoric, the local ruler, but Diablo, in his weakened state, is unable to overpower Leoric. Since Diablo is too weak to possess the king, he abandons the idea. This leaves the monarch's mind numb and his soul corrupted. He then gets Lazarus to kidnap King Leoric's son, Prince Albrecht, so that Diablo can possess and warp him, thereby attaining material form and strength. At the moment of the boy's possession, his terror is so great that the boundaries between the realms are broken and parts of Hell seep into the mortal world and take root in the subterranean labyrinth beneath Tristram. Diablo may have a body now, but he is far from his full power, so he bides his time and summons countless hordes of demons and infests the whole underground complex, claiming the new region of Hell as his own personal lair.
Soon afterwards the maddened Leoric is slain by his own lieutenants. Demons appear in the countryside as the Lord of Terror regains his strength in the heart of the labyrinth and prepares for the time when he would once again emerge to seek his brothers - Baal and Mephisto - and free them as well. It seemed to be a matter of time until the Prime Evils gained dominion over the entire mortal realm.
There are sixteen levels of the dungeon, divided into four areas. Each area has a different appearance, architecture, light level, monster mix, and musical soundtrack. The first level of each of these areas (levels 1, 5, 9, and 13) each have an additional exit leading back up to the town of Tristram.
In Single Player, these entrances are blocked until the character opens them from the dungeon side. For example, the entrance from level 13 to town is not visible at first. When the character reaches level 13 from level 12, and then finds the stairs to town, they may go up, and the entrance (a glowing crevice) opens and is available for two-way travel from then on.
In Multi-Player, the entrances to town all start in their "open" position, but with a level requirement to access them from town. A character that does not meet this level requirement will have to either gain more levels, or reach that area by completing the preceding area.
- Levels 1-4: The Cathedral. The monsters here are the weakest in the game, and shrines with curative effects abound. The Cathedral is fairly straightforward in layout, with large rooms of regular size connected by wide corridors and frequent doors and grates. Multi-Player level required: 1.
- Levels 5-8: The Catacombs. This area is more difficult than the Cathedral, and random shrines which have unknown effects begin to appear. The Catacombs are very dark, and the room layout is convoluted and confusing. Multi-Player level required: 8.
- Levels 9-12: The Caves. Powerful monsters appear here, and in Hell difficulty, the first enemies who are immune to all three magical damage types appear here (Obsidian Lords). The layout of the Caves is very complex, with rivers of lava and fences blocking the character's path in many places. Multi-Player level required: 13.
- Levels 13-16: Hell. Infested with the most powerful monsters in the game, including two more monsters which have triple immunity on Hell difficulty (the Soul Burners and Advocates). The layout is straightforward, but doors and grates do not exist in Hell, preventing the character from easily controlling his enemies' movement. The 16th level of the dungeon, lair of Diablo himself, cannot be accessed until the player has completed the quest to slay Archbishop Lazarus. Multi-Player level required: 17.
At the end of the game, the player character has again trapped Diablo in a soulstone. The hero then pierces his or her head with the soulstone, attempting to contain the Lord of Terror. However, the end of the game hints at a darker outcome. The final scene depicts the hero, swathed in a shadowy, hooded cloak. Diablo II later confirms that Diablo was indeed too strong, and possessed the hero who slew him.