You then Defeat Baal in nightmare to progress to Hell. The game will play the same in Nightmare but the loot table opens up to much better item drops/Affixes, every mob has exponentially higher life + damage, you get a -40% debuff to all resistances and some monsters start to be completely immune to certain types like Fire Immune or Lightning Immune. Then you can make Normal and Nightmare games. Once you kill him you progress to 'Nightmare'. You have to defeat the act bosses to progress up to Act 5 Baal.

In Diablo 2 you start out in Act One 'normal', you cannot change this until completing each difficulty. Necromancer - Skeletons/Clay Golem/Revive/Amplify Damage/Decrepify/Life Tap/Lower Resist/Corpse Explosion/Poison Nova/Bone Spear/Bone Prison (lots are utility)ДЄssassin - Lightning Sentry/Death Sentry/Dragon Talon/Claws of Thunder/Burst of Speed/Fade/Shadow Master/Shadow Warrior/Cloak of Shadows/Mind Blast/Psychic hammer (lots are utility)ДЄmazon - Charged Strike/Lightning Fury/Penetrate/Pierce/Valkyrie/Exploding Arrow/Freezing Arrow lots are utility)Дruid - Tornado with Hurricane/Grizzly/Oak Sage/Shock Wave/Fury/Fissure with Molten boulder (fire is weak compared to Wind)

Paladin - Blessed Hammer with Concentration/Holy Shield with Smite/Zeal with Fanaticism (blessed hammer is strongest, almost nothing immune even in Hell :D)Д«arbarian - Frenzy/Double Swing/Whirlwind with Concentrate/Berserk/Leap/Battle Orders/War Cry/Battle Cry/Find item/Increased Speed/Natural Resistance (frenzy is best. Sorceress - Blizzard/Frozen Orb/Meteor/Fireball/Firewall/Lightning/Static Field/Teleport/telekinesis (cold is best, lite is great w/ Infinity, fire is worst because of immunes) Your +skills from gear will increase them later on. *Warning: Some are only for Crowd Control or Support, typically only put 1 or a couple points into these skills tops. Some Endgame Viable Skills for Every Class. There are quite a few weak and mediocre skills in Diablo 2 so choose very carefully or you may have to re-create your character if you've already used your first Den of Evil Respec: Only focus on maxing out 1-2 main attack skills and their synergies or else your skills will become too weak later on. You can max out each skill with 20 points, although bonuses to skills from gear pushes this beyond 20. After that the only way to respec is to farm the Act bosses in Hell difficulty to create respec tokens. You Only Get 1 Skill Respec Per Difficulty for completing the Den of Evil quest. Also a refresher if you're coming back after a long break.

Diablo 2 quest guide