To use the Adamantine Forge in Baldurs Gate 3, you need to insert a mould and mithril ore into the forge. Specifically, you should insert a mould into the mould chamber and place mithril ore into the crucible. Once you have inserted the mould and mithril ore, you need to release the lava valve and pull the forge lever. Keep pulling the forge lever to drop the giant hammer down on the crucible. A finished item will eventually pop out of the mould chamber, which will let players do the process all over again. It is important to note that you need to defeat Grym, the large golem protecting the forge, before you can use the forge.