This quest is triggered either upon freeing the spirit of Izual, or by entering the River of Flame area. The quest is given by Cain and revolves around the character breaking Mephisto’s Soulstone on the Hellforge, which can be found further into Hell.
Like the first quest of the act, this act is also optional, but again it does offer a good reward upon completion.
You will find the Hellforge somewhere in the River of Flame. As you explore this area, you will come to a fork in the path; an angel will be present on one path. The Hellforge is not on the same path as the angel, so go back and explore the other path instead to find the Hellforge. The angel won’t try to stop you, however, if you decide to continue along the chosen route towards the Chaos Sanctuary.
In order to break the Soulstone you will first have to defeat its protector, a demon called Hephasto. This boss monster was one of the toughest of the game prior to the 1.08 patch, but even since he’s still a fearsome opponent. You need to kill Hephasto to retrieve the Hellforge Hammer.
With the Soulstone placed on the Hellforge, shatter it using using the hammer – you’ll need to click the Soulstone three times to destroy it – and you’ll free lots of spectral spirits who were trapped in the stone. By way of reward, shattering the stone yields a collection of gems, and also a rune if you are playing the expanded version of the game. The types of gems dropped are randomly generated, but will always consist of two normal gems, one flawless gem and one perfect gem. If you are playing multiplayer, four gems will be dropped for each character in the game that has not yet completed the Hellforge quest.
Before you go any further into the game, it’s wise to return to town to restock with potions and repair equipment, or to equip new gear. The next quest is the final quest of the act and is your showdown with Diablo.