Nightmare is the difficulty level following Normal. Although you will have the creatures with the same appearance and the same quests as in Normal, they will be much harder and a few might use more spells.
For having access to the Nightmare difficulty you must meet one of the following requirements:
- If you have Lord of Destruction (game), you must have defeated Baal at Normal difficulty;
- If you do not have the expansion, you must have defeated Diablo at Normal difficulty.