Act IV Quest 2: Hell's Forge - Diablo 2
Quest Location: River of Flame
Quest NPC: N/A
Quest Goal: Use the Hellforge Hammer on Mephisto's Soulstone at the Hellforge
Reward: Many Gems and some High Runes
The Hellforge is the second quest in Act 4 and begins at the end of Act 3 after you defeat Mephisto and pick up Mephisto's Soulstone. What you need to do for this quest is to bring Mephisto's Soulstone to the Hellforge which is found in The River of Flame, the zone right before Chaos Sanctuary that houses Diablo.
Defeat Hephasto near The Hellforge to acquire the Hellforge Hammer which you'll need to equip and use on the Hellforge in order to destroy Mephisto's Soulstone. Once you've done this, the quest will be completed. Your reward for completing this quest is whatever Gems are dropped on the ground after destroying Mephisto's Soulstone. How many Gems you receive depends on how many players are in your game or how many players you have your game set to.
Below are two screenshots that show you how many Gems dropped for me when I had all 8 players in my game on Nightmare difficulty.