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 for the Archbishop Lazarus and Diablo quests, which are the keys to finishing the game. Quests add background lore details and most give valuable rewards (even if only when just reselling).
Some quests appear only in Single Player mode and those that appear in Multiplayer mode 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 selected quests are given in a newly started 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 Supply||The Butcher*||Magic Rock||Zhar the Mad||Warlord of Blood||Chamber of Bone|
|Skeleton King*||Ogden's Sign||Arkaine's Valor||Black Mushroom||Lachdanan||Archbishop Lazarus*|
|Gharbad the Weak||Halls of the Blind||Anvil of Fury||Diablo*|
It is easy to immediately tell which Group 1 and 2 Quests you have upon starting a new game. If the fountain water next to Deckard Cain is murky, you will get the Poisoned Water Supply Quest; however if the fountain water is clear, you will get the Skeleton King Quest.
Similarly, if The Wounded Townsman is missing from the entrance to the Cathedral, that means that you miss out on The Butcher Quest and get the other two quests instead: Ogden's Sign and Gharbad the Weak. If the wounded townsman is there, then you will get the Butcher quest and either one of the Ogden's Sign or Gharbad the Weak Quests (you will need to further explore the cathedral to find out which one you will get).
Because certain quest rewards are better suited for certain classes, you may restart the game until you get the quests that you wish for.
The quests with an * (asterisk) 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 roaming freely and Archbishop Lazarus on level 15, standing behind the altar with the sacrificed child in a specially designed large room.