When players manage to reach the Adamantine Forge in Grymforge, they'll need at least one mold and a chunk of mithril ore to start the process. Players will also need to beat what's easily the toughest boss battle in the entire Underdark region of Baldur's Gate 3.
Baldur's Gate 3: The Adamantine Forge (Grym) Boss Guide
The adamantine golem, is an incredibly tough monster in Baldur's Gate 3, but a couple tricks can make him beatable.