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 Baal 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 (2,000,000-1,000,000), that is, 5% or 10% of 1,000,000.
Retrieving your corpse where you died in Nightmare and Hell restores 75% of any experience lost at death.
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 20% chance to drop an Exceptional piece of Armor or Weapon, 40% for special Monsters. They also have a 30% chance to drop an Elite piece of Armor or Weapon, 40% for special Monsters.
Unique and Champion Monsters
Unique and Champion Monsters gain a 66% damage bonus in Hell.
Monsters
Monsters are more difficult in Act V and Hell Difficulties than Nightmare Difficulty by improving the monsters stats (hit points, defense) and by boosting their AI/behavior. They make decisions more often and move faster. Monster damages generally increase also.
The hit points of Barbarian Soldiers in Act 5 are reduced.
Monster levels, hit points, Armor Class, Experience, Damage, Chance to Hit, and Resistances all go up.
Monster skill levels go up +7 in hell.
Unique Monsters gain two bonus abilities.
Monsters have more immunities.
There are more Unique Monsters in Hell difficulty.
Nihlathak's Corpse Explosion deals 20% damage in Hell.
Monsters populations are more randomized including guest monsters from other acts.
Player Resistances
Player and Hireling resistances are lowered by 100 in Hell Difficulty.
Hireling Damage vs Act Bosses
Hirelings and Summons do 25% damage to Act Bosses in Hell.
Life/Mana Stealing
Are cut 2/3 in Hell.
Freezing, Chilling, AI affecting Curses (Confuse, Attract, Dim Vision)
Are cut 3/4 in Hell.
Static Field
Static Field cannot bring a monster's life lower than 50% in Hell.
Other
Many areas will now have much larger maps. The Forgotten Tower, The Catacombs, The Durance of Hate and the Worldstone keep are a few examples of these areas.
Rewards for defeating Hell Difficulty
New Titles
Normal Mode: Hell Difficulty: Patriarch, Matriarch
Hardcore Mode: Hell Difficulty: Guardian