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. Furthermore, while in Normal difficulty dying loses you gold, in Nightmare you will also lose experience.
The full list of changes are these:
- Increased monster levels and stats (hit points, defense, damage, etc.)
- Monster resists are often buffed, making them more resilient to damage.
- Player and Hireling resistances are reduced by 40.
- Death results in a loss of 5% of the experience required to reach the next level. Retrieving your corpse restores 75% of the lost experience.
- Hirelings and Summons deal 35% damage to Act bosses.
- Duration of Freezing and Chilling effects and AI-affecting curses are reduced by 1/2.
- Life and Mana Leech effects are reduced by 1/2.
- Static Field cannot reduce a monster's health below 33%.
- Many area maps are increased in size.
- Act V has additional "Guest Monsters" from other acts to increase the difficulty.
For having access to the Nightmare difficulty you must meet one of the following requirements:
- If you are playing an expansion character, you must have defeated Baal at Normal difficulty;
- If you don't, you must have defeated Diablo at Normal difficulty.
- Hardcore: Male will be Duke and Female Duchess
- Softcore: Male will be Lord and Female Lady
If it is an expansion character, you must kill Baal in Act 5 to gain a new title.
- Hardcore: Male and Female will be Conqueror
- Softcore: Male and Female will be Champion