Nightmare (difficulty)

Nightmare is the difficulty level following. Although you will have the creatures with the same appearance and the same quests as in Normal, they will be much harder and dish out more damage.

Diablo/Hellfire
Nightmare is present in Multiplayer of Diablo, as well as in all of Hellfire. Both have varying differences with the monster changes, but a list below shows changes present in both cases:
 * Monsters will deal more damage per hit, and they possess at least 3x the health they would have possessed in Normal
 * Players will need +80% to-hit and +80 Armor Class to dish out/avoid damage like they do in Normal.

Diablo Only

 * While the fact remains that monsters receive at least x3 the HP they would possess in Normal, tests have shown that they receive +1 to their HP after the modifier

Hellfire Only

 * Monsters receive +50 to their HP capacity after the initial x3 Difficulty bonus for Nightmare, making foes hardier than they are in regular Diablo

Diablo 2/Lord of Destruction
Keep in mind that while death in Normal will just rob you of a sum of gold from the Stash, Nightmare will also begin taking experience away for dying. These changes, of course, do not affect Hardcore heroes, since death is permanent in that mode.

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 in the expansion. Regular Diablo 2 players only suffer -20% all resists, but have less means to combat them- no charms, no runes, jewels and no Scrolls of Resistance.
 * 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.

If a character accomplished the last Quest Act 4 (killing Diablo) in non-expansion you will gain a new title.
 * 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