Instructions
Complete quests and defeat enemies in a "Diablo" multiplayer campaign of any difficulty level until your character has reached level 30 to unlock the "Hell" option.
Press "Esc" to bring up the game's main menu. Click on the "Options" button to change your campaign settings.
Select "Hell" as the difficulty level to enable Hell mode in your current campaign. Hell mode will remain enabled for the duration of the campaign or until you further modify the difficulty settings. Click the "Multiplayer" button in the game's main menu and select the Hell Difficulty after exiting your current game to begin a new "Hell" campaign.