Quest 1 - The Fallen Angel

Quest Giver: Tyrael the Archangel.
Begin by: speaking with Tyrael at the beginning of the Act.
Quest Location: Around the perimeter of the Plains of Despair.
Quest Reward: +2 skill points to distribute.

Act 4 is the shortest act in the game. There are only four areas plus the Chaos Sanctuary. Speaking with Tyrael, he mentions there's a tortured soul in the Plains of Despair, a fallen angel named Izual. You're to find him and end his suffering.

Follow the steps down from the Pandemonium Fortress (you have to feel sympathy toward those two merchants standing there... probably not much business passing through hell) into the first area. Search around the perimeter for the only exit from the area, stairs leading to the Plains of Despair. This is a similar area only with new enemies. One of them is Izual and he'll find you if you run around the perimeter. He's got an absolute ton of hit points, so expect him to take awhile. He also has a cold attack that can slow you down. But he's not the hardest hitter, so not altogether very tough. If you're an Amazon, cast decoy and Valkyrie and attack him as he whacks on them. When he's finished, speak with his spirit to end the quest, and then see Tyrael for a reward.


Quest 2 - The Hellforge

Quest Giver: Cain, master of knowledge and the Sean Connery accent.
Begin by: speaking with Cain after completing Quest 1 or entering the River of Flame.
Quest Location: Hellforge, River of Flame.
Quest Reward: Four random normal-perfect gems, and a rune if you have the expansion.

After completing Quest 1, talk with Cain. He'll tell you it's time to destroy Mephisto's Soulstone, by breaking it with a hammer at the Hellforge. You'll want to travel first to the third area, called City of the Damned, and activate the waypoint teleporter. The entrance to the River of Flame is always nearby.

The River of Flame is a maze through lakes of lava, with the Hellforge being on either a point on the North or South end. Straight to the east is the third and final waypoint of the act, near the entrance to the Chaos Sanctuary. As you approach the forge, you'll fight some lesser creatures, and then the huge unique monster Hephasto attempts to rip you apart. He's very similar to the other smiths you've fought during the game, Griswold and the one in the Monastery in Act 1, because he hits for a lot of damage and will kill you quickly unless you're careful. He's also very quick and tough to outrun in pre-patch 1.08 games. But since then, he's a little easier and doesn't run as fast. Like the other tough melee monsters, if you're a ranged character, try and run around to the other side of a walkway where he won't be able to find you and kill him from there. Or else, have some minions ready to take the assault.

When he's dead, pick up the Hellforge Hammer he drops, proceed to the forge and equip the weapon. Make sure you have the soulstone in your inventory, and activate the hellforge. The stone will appear on top. Now hit it with the hammer and claim your reward as the soulstone is smashed into shards. If you haven't already, talk to Tyrael for Quest 3.


Quest 3 - Terror's End

Quest Giver: Tyrael.
Begin by: speaking with Tyrael after completing Quest 1 or entering the Chaos Sanctuary.
Quest Location: The Chaos Sanctuary.
Quest Reward: Diablo's drops (no not those kind), a new title if you don't have the expansion, plus a portal to Act 5 if you have the expansion.

The final quest in Diablo II, talk to Tyrael again after completing Quest 1 and he'll send you on a bounty hunt, the bounty being humanities' salvation. Quest 2, the Hellforge, is optional, as are many of the quests in Diablo II. You can skip it and proceed directly to Diablo after you kill Izual if you wish.

The Chaos Sanctuary is past the waypoint teleporter in the River of Flame. You'll run through a small maze to the steps inside the "Sanctuary". Look out for Obsidian Knights when you're running through the easy maze. They'll attack out of range, so either throw some minion dummies at them, or close in on them quickly. You can usually avoid their range attacks, but when they hit, they hit hard.

Upon entering, you'll travel through a long corridor with a couple sets of Obsidian Knights and other creatures blocking your way into the central area where Diablo eventually spawns. You're objective is to activate each of the five seals spread out around the North, South and East points of the level. They are always the same, and three of them will spawn a Unique boss with minions as soon as you activate it. Beware. It's advisable to throw a town portal up at the beginning of the steps to the Chaos Sanctuary in case there's trouble. You might also be making several trips back to sell the great equipment that's possible to retrieve from all the boss encounters.

There are two seals to the North, one to the East, and two more to the South. Only one of the two seals at the North and South will spawn a boss and minions, and the one at the East will always spawn a very tough Obsidian Knight boss and minions that can make short work of unprepared adventurers. When you activate the fifth and final seal, the level grows darker and you'll watch Diablo spawn in the central area on the pentagram. This is when you should throw a town portal off to the side of one of the Sanctuary points, and get prepared. Leave it up so that you can always come back if (most likely when) you die.

Diablo is the toughest creature in the game. He's even much tougher than Baal in the expansion. The easiest way to beat him is to get close so that he melee's you, his weakest attack. When he spits his white lightning stream out, it's the roughest attack in the game and he'll spin around as he does so. Try and avoid this at all costs. It will wipe out your Valkyrie and decoys if you're the Amazon, and most likely any minions you have. Have as much lightning resistance as possible before you fight Diablo. When he's dead, he usually has good drops, so hopefully you'll get something useful. Return and speak with Tyrael to end the game, or to advance to Act 5 in Lord of Destruction.