Unlocking some of the Adamantine Forge weapons and armor in is an excellent way of gearing your characters up with well-crafted items in the game's early stages. The Adamantine Forge is an important part of Act 1, though accessing it is completely optional. It is a part of the Underdark’s Grymforge region, where you have to deal with True Soul Nere. If you continue to explore Grymforge and delve deeper, you may encounter the Adamantine Forge, some scattered items, and the area’s special boss, Grym.
Once you have beat Grym in , you will have unlocked access to the Adamantine Forge, a special location allowing you to forge limited items. The Adamantine Forge items are rare pieces of gear that can offer a significant bump to builds and are some of the best equipment you can obtain early in the game. These items are so good, in fact, that you will likely continue to use some of them through Act 2 and into Act 3.
The big issue with the Adamantine Forge items is that they are extremely limited. Per run, you can only forge two of the six items available in the forge. To make these pieces of gear, you will need to collect special items called Moulds. For instance, you will need to have the Longsword Mould in your inventory to forge the Adamantine Longsword. These Moulds are scattered across various locations in the Grymforge. In addition to Moulds, you will need one Mithral Ore per piece of equipment you want to forge in .
The scarcity of Mithral Ore limits each run to two items from the Adamantine Forge in . Mithral Ore can be found in two Mithral Veins inside the Grymforge map. These Mithral Veins are represented by a large stone with blue accents. The first is on a path to the left of the stairs you take down toward the Adamantine Forge, across a river of lava. Its coordinates are approximately X: -557, Y: 278. All you need to do is break the Vein in and collect the Mithral Ore from the floor.
Before crossing the river, you will be ambushed by a party of Magma Mephits.
Read more on screenrant.com