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When players manage to reach the Adamantine Forge in Grymforge, they'll need at least one mould 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.
Grym, as Baldur's Gate 3 calls it, is an Adamantine golem that emerges onto the forge once players spin the lava valve. Grym resists or nullifies almost every damage type, and even then that's only when he's superheated from walking through lava. This makes it hard to deal any damage to his 300 hit points, but thankfully there's one environmental damage source Grym is critically weak against.

Baldur's Gate 3: All Adamantine Forge Mould Locations In Grymforge
Explore the Underdark and discover relics used for crafting equipment at the Adamantine Forge.
Updated on April 21, 2024, by Nahda Nabiilah: The Adamantine Forge is one of the many essential areas players need to visit while in the Underdark. The forge can reward a decent weapon or armor in Baldur’s Gate 3 if Tav provides the required materials: the mould and mithril ore. Unfortunately, not everything is as easy as it seems, since the forging process triggers the Grym boss fight, which is a powerful enemy that’s almost immune to every single damage in the game.