The Underdark merchant Harkstone gives PCs a chance to explore and get rich with an offer to recover a large chunk of meteoric mithril from a merchant's abandoned underground home. They discover that the merchant's home isn't exactly abandoned, nor are they the only treasure-seekers.
The PCs travel from Sigil to the gate town of Fortitude to find a portal leading to the land of Zakhara on the Prime Material world of Toril, and discover that the portal is guarded by a mysterious and malevolent force.
