Hell's Forge is the second quest of Act IV. It's initiated by talking to Deckard Cain. It's completed after players smash Mephisto's Soulstone at the Hellforge.
The Hellforge
Further Tips and Information
The fury of Mephisto's Soulstone unleashed.
Find the City of the Damned. Somewhere in this City is a staircase leading down to the River of Flame (you'll find a Waypoint near the stairs). Search the River of Flame until you find the Hellforge; if you find a Waypoint located near a corridor with of demon statues you have to turn back. When you approach the Hellforge the quest log should appear and you should encounter Hephasto the Armorer, a Unique Overlord boss similar to The Smith. Much like The Smith, Hephasto has a powerful melee attack and moves fast. It's best to lure him away from the forge and kill him when he's alone; it might be a tricky fight however. When he dies he should drop the Hellforge Hammer. Clear the area near the Forge and equip the hammer. When you click on the Hellforge, Mephisto's Soulstone should disappear from your inventory and appear on the forge. Attack the forge; after several swings the Soulstone should shatter into several fragments and a storm of souls is unleashed.
Several gems will drop and a rune; a nice reward for completing the quest. Notice that the Hellforge Hammer is gone. Return to the Pandemonium Fortress and speak to Cain and Tyrael, who will present you with a final task...
| Diablo II and Lord of Destruction quests |
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 Diablo II quests
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Act I — Den of Evil • Sisters' Burial Grounds •
Search for Cain • The Forgotten Tower • Tools of the Trade • Sisters to the Slaughter |
 Diablo II: Lord of Destruction quests
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| Act V — Siege on Harrogath • Rescue on Mount Arreat • Prison of Ice • Betrayal of Harrogath • Rite of Passage • Eve of Destruction |