In Diablo I, the quests are a series of missions (fewer in multi-player) on which the character is sent, and which offer advantages when completed. None of them need to be completed, except the Archbishop Lazarus and Diablo quests, which are the key to finishing the game. Quests add background lore details and most give valuable rewards (even if only when reselling).
Some quests appear only in single player and those that appear in multiplayer are simplified versions of the single player ones. Quests that appear in single player alternate between different games to make for better replayability. They are sorted into groups from which only some quests are given in a game.
| Group 1 | Group 2 | Group 3 | Group 4 | Group 5 | Group 6 |
|---|---|---|---|---|---|
| 1 out of 2 | 2 out of 3 | 2 out of 3 | 2 out of 3 | 1 out of 2 | Always appear |
| Poisoned Water | The Butcher* | Magic Rock | Zhar the Mad | Warlord of Blood | Chamber of Bone |
| Skeleton King* | Ogden's Sign | Valor | Black Mushroom | Lachdanan | Archbishop Lazarus* |
| Gharbad the Weak | Halls of the Blind | Anvil of Fury | Diablo* |
The quests with an * are available in multiplayer, though they are much simpler than their single player counterparts, as they do not have special levels, the Skeleton King appearing on level 3 and Archbishop Lazarus on level 15, near the altar with the sacrificed child.