The Hellforge

You must destroy the

Mephisto's Soulstone you got from

Mephisto on

The Hellforge, a blacksmith's forge in Hell.

This is the forge

Izual was attacking to prevent the completion of Shadowfang when he was captured, in the Diablo mythology.

At

The Hellforge you'll find

Hephasto the Armorer. You must kill him to make him drop the

Hellforge Hammer, which you should then equip.

With

Mephisto's Soulstone in your inventory, and the

Hellforge Hammer equipped, click on the anvil until the soulstone is destroyed.