It should be no surprise that, with all its details, houses many secrets. Larian Studios has filled their award-winning game with hundreds of pieces of armor, some requiring the felling of powerful bosses or monumental sums of gold to acquire. Much like many of these items are magical by nature and are well-sought after during a playthrough, but very few can be forged using a seemingly useless item.
[Warning: The following contains spoilers for Baldur's Gate 3]
An infernal alloy, not to be confused with infernal iron, has no other functions other than being used as a material for the forge. There is no quest associated with the infernal alloy, unlike the sussur tree bark used to forge a masterwork weapon in Act 1. With it sharing artwork with infernal iron, but lacking its use in Karlach's questline, it can be easily overlooked as nothing but a salable trinket to earn a pretty penny, yet it is the key to creating the Flawed Helldusk armor set, one of best.
The player must have reached the Underdark toward the end of Act 1. In the Underdark, there is a subsection called the Grymforge, where the player can interact witha duergar stonemason called Kith. Strangely enough, Kith is relatively polite to the player character and will be in the middle of investigating a part of a stone wall that has been reduced to rubble. When the player interacts with him, he will ask what the player sees, leading to three ability checks that they will need to pass.
All three successes are required for Kith to reward the player with the Infernal Alloy. If the player is unable to pass the checks, then they have the option of simply purchasing the alloy from the stonemason vendor. It should be noted that the player should interact with Kith before the encounter with True Soul Nere, as the duergar in the Grymforge will either leave or become hostile after that encounter. There is no other opportunity to get Infernal Alloy.
Infernal Alloy is the only type of infernal metal that appears once
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