Mephisto's Soulstone

Mephisto drops Mephisto's Soulstone in Durance of Hate Level 3. In Hell's Forge, Tyrael asks you to smash Mephisto's Soulstone with the Hellforge Hammer on the Hellforge in the River of Flame. Smashing Mephisto's Soulstone rewards you with gems and a rune: see the Hellforge.