Diablo 2 Walkthrough -> Act 4
Quest 1 - Fallen Angel
Talk to Tyrael and Kain. They'll quickly bring you up to speed. Tyrael will give you your first quest. Izual, a fallen angel, escaped the gates of Hell several years ago and was caught. Since then, he's been enslaved by the Prime Evils; your task is to free him. Of course, since this is Diablo, you'll have to free him by killing others.
Descend the Outer Steppes and clear this level. Izual is clearly out of shape; he's frozen solid. The guys you meet there are the weakest you'll encounter in Hell, so if you're constantly dying, you might want to stay here and level up.
From the Outer Steps, you'll need to exit through the archway that leads to the Plains of Despair. You'll find Izual there:
Fight him, defeat him, and then speak with him. Afterward, return to the Fortress and speak with Tyrael.
Quest 2 - Hell's Forge
Kain will remind you that you need to destroy Mephisto's Soulstone in the Hellforge. The forge is located beyond the River of Flame, which adjoins the City of the Dead, just below the Plains of Despair. You'll come to a crack in the road, not far from the Rover of Flame.
If you encounter an angel who tells you that the Hellforge is in the opposite direction, go back. Kill the little freak named Hephasto and anyone else nearby:
Then you'll see the Hellforge Hammer fall. Equip it and smash the Soulstone for high-quality gems. Return to town, talk to Kain, and prepare for battle.
Quest 3 - Terror's End
It's time to take on the Big Guy himself... Is that HOT enough for you? You'll need to head back to the River of Flame and approach the Chaos Sanctuary. Be sure to activate the waypoint at the River. The Sanctuary is teeming with bad guys, and every single one of them is out to kick your ass.
Once you've made your way into the main hall, activate the five seals to free Diablo:
In fact, we highly recommend killing everyone in this room first, as three of the five seals will spawn extremely nasty unique NPCs after you activate them. Once you've activated all the seals and killed all the NPCs, the Devil will appear.
If you're playing with multiple players, now's a good time to check if you have room to store extra gear. After you kill the Devil, you'll have ninety seconds left in the game.
The Devil is similar to Godzilla, and he uses similar moves, like when he's trying to crush your ass. He has a variety of ranged attacks, the most terrifying of which is a bolt of lightning that instantly destroys anyone in its path:
For combat characters, the Devil will be the most difficult challenge. His lightning is terrifying, but he warns you of it half a second before it strikes.
Mages: don't turn your back on him and let your magic show what you're capable of. Don't get too close, and you'll kill him easily. When he falls, dead, quickly loot his gear and then watch the final cinematic. Congratulations, you've defeated Diablo again! Go to the act 5 of Diablo 2.
See also:
Act 3: Diablo 2 Walkthrough (Lod)
Dark Energy: Half-Life 2