All games begin on Normal Difficulty. You can create and join games on the next difficulty level once you have defeated the game on Normal Difficulty. You can create and join games on Hell difficulty once you have defeated the game on Nightmare Difficulty. This is the case for each of your characters.

Once you have beaten the game on Normal Difficulty, you can create Hardcore Characters.

Level Recommendations
If your goal is experience, you may not want to jump into Nightmare difficulty right away after defeating the game on Normal Difficulty. The Monsters in Nightmare start at level 26 and quickly increase in level. You'll find that these monsters have a better chance to hit you and you'll have a harder time hitting them because they are higher level than you when using Melee attacks. You will probably not receive as much experience as staying in Act IV of Normal Difficulty for a while longer. Also Nightmare has the additional risk of losing experience when you die so you may lose all of the experience you have gained recently with a few deaths on Nightmare Difficulty. It's recommended to immediately complete the Den of Evil quest again on Nightmare Difficulty as soon as you have defeated Normal Difficulty for the reward and then hop back into Act IV.

To be safe stay in Act IV as long as you are earning good experience. You can still get good experience in Act IV until at least Level 35, and you can stay longer in this area by joining games with more players which will increase the amount of experience a monster gives but will also increase their hit points. You can test the experience in different areas by comparing the amount experience you get within a given time using a watch or stop watch.

Note that once you have defeated Normal Difficulty you will only be able to see Nightmare games on your games list for Realm Characters. You can, however, still create Normal Difficulty games. Characters that have defeated Nightmare, will only see Hell Difficulty games on their games list for Realm Characters but can create games on any difficulty or join games on any difficulty if you type in the name of the game.

  Normal Diffriculty [top]

Normal Difficulty is the default Difficulty level. Only by defeating the game on Normal Difficulty level can you unlock higher difficulty levels and Hardcore Mode. You do not lose experience when dying on Normal Difficulty level so it's recommended that you stay and level up there as long as you can. Before you can advance on to Nightmare Difficulty you must defeat Diablo on Normal Difficulty.

Rewards for defeating Normal Difficulty

You can now create and join Nightmare Difficulty games. You will now see Nightmare Games on the games list for Realm Characters.

You can now create Hardcore Characters.

New Titles
Normal Mode: Normal Difficulty: Sir, Dame Hardcore Mode: Normal Difficulty: Count, Countess

  Nightmare Difficulty [top]

Once you have defeated the game on Normal Difficulty only Nightmare Difficulty games will appear on the games list for Realm Characters.

How do I start or join a Nightmare Game?
You must first defeat Diablo on Normal Difficulty.

Experience Loss
When you die in a Nightmare Difficulty game you will lose 5% of your experience. The experience loss (5% in Nightmare and 10% in Hell) is applied to the experience difference from the current level to the next level. So, if you achieved level X at experience 1,000,000 and you would achieve level X+1 at experience 2,000,000, then the penalty is 5% or 10% of (2,000,000-1,000,00), that is, 5% or 10% of 1,000,000.

Player Leveling
Players will level up faster (from levels 26-50ish) in Nightmare.

Items
Players will have a better chance of getting magic enhanced items. Monsters will also have a slim chance (5%) to drop an Exceptional Piece of Armor or Weapon with an increased chance for special Monsters (10%).

Resistances
Player resistances are lowered by 20 in Nightmare.

Freezing, Chilling, AI affecting Curses (Confuse, Attract, Dim Vision)
Are cut in 1/2 in Nightmare.

Monsters
Monster hit points, Armor Class, Experience, Damage, Chance to Hit, and Resistances all go up.

Monster level goes up, according to this formula :

Nightmare Monster Level = Monster Level + 25
So, a level 2 Skeleton in the regular game is level 27 in Nightmare.

Monster skill levels go up +3.

Unique Monsters gain one bonus ability.

Rewards for defeating Nightmare Difficulty

 

You can now create and join Hell Difficulty games. You will now see Hell Games on the games list for Realm Characters.

New Titles
Normal Mode: Nightmare Difficulty: Lord, Lady
Hardcore Mode: Nightmare Difficulty: Duke, Duchess

  Hell Difficulty [top]

Once you have defeated the game on Nightmare Difficulty only Hell Difficulty games will appear on the games list for Realm Characters.

How do I start or join a Hell Difficulty level game?
You must defeat Diablo on Nightmare Difficulty.

Experience Loss
When you die in a Hell Difficulty game you will lose 10% of your experience. The experience loss (5% in Nightmare and 10% in Hell) is applied to the experience difference from the current level to the next level. So, if you achieved level X at experience 1,000,000 and you would achieve level X+1 at experience 2,000,000, then the penalty is 5% or 10% of (2,000,000-1,000,00), that is, 5% or 10% of 1,000,000.

Player Leveling
Players will level up faster (from levels 51-75ish) in Hell.

Items
Players will have a better chance of getting magic enhanced items. Monsters have a 10% chance to drop an Exceptional piece of Armor or Weapon, 20% for special Monsters.

 

Monsters
Monster hit points, Armor Class, Experience, Damage, Chance to Hit, and Resistances all go up.

Monster level goes up, according to this formula :

Hell Monster Level = Monster Level + 50
So, a level 2 Skeleton in the Normal game is level 52 in Hell.

Monster skill levels go up +7 in hell.

Unique Monsters gain two bonus abilities.

Player Resistances
Player resistances are lowered by 50 in Hell Difficulty.

Freezing, Chilling, AI affecting Curses (Confuse, Attract, Dim Vision)
Are cut 3/4 in Hell.

Rewards for defeating Hell Difficulty

 

New Titles
Normal Mode: Hell Difficulty: Baron, Baroness
Hardcore Mode: Hell Difficulty: King, Queen