Hellforge

From Diablo Wiki

The Hellforge is a magical forge found in the Burning Hells which has the power to destroy a Soulstone. The Hellforge is protected by an Overlord called Hephasto the Armorer, who holds the Hellforge Hammer required to smash the Soulstone. During Diablo II, your hero completes a quest named Mephisto's Soulstone, where he/she smashes Mephisto's Soulstone using the Hellforge and the Hammer.

The Hellforge Quest

LoD players will find one of the most rewarding quests, the Hellforge quest, in the game in Act IV. To complete the quest, a hero must destroy Mephisto's Soulstone that was obtained earlier using Hephaisto's Runeforge Hammer. The smashing of the soulstone causes various types and qualities of gems to drop, but the ultimate prize is the single rune that falls as well. The difficulty in which the quest was completed dictates what the rune can be:

This quest reward is the only occassion in the game where the normal dropchance of a rune is ignored. Instead, each different rune in proper difficulty drop range has a 9.1% chance of droping. It is important to understand that this is no guaranteed free ticket for getting an HR, although it may happen with some luck. Also, because Hell drops only allow runes up to Gul, the eight rarest runes cannot be selfobtained any other way than finding one by sheer luck. For ladder players, the most effective way of getting Mid and High runes by yourself is through Hellforge rushing characters and collecting the quest reward until you have the proper runes needed for cubing.